The Dilemma
Generally speaking, every post I've read (whether its on [H]ardforums, TomsHardware, Anandtech, TechSpot etc etc...) asking about whether they should go the Single GPU route or Dual GPU route is always bombarded with posts recommending the Single GPU route citing issues with Dual GPU setups and what not. Although I would say that is the case in most situations, I personally feel that OP's of these threads should really tell more about their situation as I have personally experienced both CF/SLI and feel that the choice needs to be determined by a case by case basis.

Background
So first off I've lived with 6850 Crossfire for about 2 years, but have recently switched to a single GTX 670 and have been staying with that for a couple of months. Prior to this I've also run 4850x2's in Crossfire and 9800GT's in SLI. However during the 4xxx ATI and 9xxx Nvidia days, I can safely say that it was not worth the cost--but times have changed, the technology has improved and I feel that it is a true contender for frames per dollar. Unfortunately I started this blog after I switched to the GTX 670 so I didn't take any screenshots to show hard proof to backup what I say, but there's more than enough reviews and benchmarks from the plethora of Tech Review Sites that currently exist to give you a better idea of the multi-GPU Technology.

Content
First I'm going to start off with the issue's I've experienced with Crossfire (most of this stuff applies to SLI aswell but SLI DOES differ from Crossfire in some aspects for better or worse). Then I'll cover what benefits of Crossfire I've experienced and then conclude with a sort of a case by case basis on whether multi-GPUs are for you or not.

Intricacies of Crossfire / Multi GPU

• Not all games support Multi GPU setups
• This is probably the biggest issue of Multi-GPU setup's and the most common reason people do not recommend CF/SLI. Isn't it wasteful to spend money on something that only works most of the time? 
• In some cases, you actually get WORSE performance for applications that cause negative CF/SLI scaling.
• Graphical Glitches in games
• The only game that produced this issue for me was The Secret World using the Dreamworld Engine produced by FUNCOM. 
• The Game had serious flickering issues with shadow and lighting, and text appeared corrupted and unreadable. Both these issues occurred only in DX11 mode. DX9 did not have the issues but Crossfire scaling was nowhere as good as in DX11 mode.
• Greater reliance on drivers
• Multi-GPU setups are seriously reliant on AMD/Nvidia to chug out profiles and modifications to their drivers to utilize the GPU's. This propagates an issue with BRAND CHOICE.
• NVIDIA for the most part seems to have a better track record with pushing out drivers (as well as SLI Profiles). During my 6850 days I would wait months for a driver update from AMD while NVIDIA usually provides an update every month or so.
• The most recent issue I dealt with was when I was tried out Final Fantasy XIV in beta. SLI was ready a month or two prior to Crossfire supporting the game.
• Requires an SLI/CF Compatible Board
• This is one of the reasons multi-GPU setups need to be looked at on a case by case basis. If a person is planning to purchase a motherboard that supports it or already has a mobo that already supports it,  this is not really a con.
• Most mid-ranged/sub-enthusiast boards have multiple PCI-E 2.0/3.0 slots that support atleast x8/x8 (~$100+ mobos)
• Generally people get these boards for more power phases for better OCing capabilities and more general features on their motherboards.
• Note: I read a thread on the EVGA forums with benchmarks showing PCI-E 2.0 as a bottleneck for ultra-high resolution gaming. As a result motherboards with multiple PCI-E 3.0 slots is something to look at if you plan to do this. Then again if you have many expensives monitors, you probably already have an high-end enthusiast mobo already which does have multiple PCI-E 3.0 slots ($200+ motherboards) 
• Greater Power Consumption
• More processing power -> More power consumption.
• I think this issue goes either way. For example in my case, 6850's in crossfire actually use less power on load than a single GTX 480. However you could just say that would be an embarrassment on NVIDIA's end with their Fermi architecture. 
• Greater Heat Production
• Again another obvious point, but depends if the extra performance is worth the extra heat to the user.
• Primary GPU Runs hotter by ~10C
• This can be caused by multiple things:



From Guru3D




From Hardware.info



• The two Cards are too close together which results in less air flow to one of the GPUs since the PCB of the second card inhibits the air from entering the intake of the first card. 
• This usually only applies to motherboards that put PCI-E slots too close together
• Most MicroATX motherboards have only have a single PCI-E x1 slot in between each x16 slot to fit the smaller form factor, somewhat unavoidable for smaller rigs. 
• You can try cramming a small piece of styrofoam reduce the impedance of air flow--this mitigated the issue for my friend so even a couple millimeters helps
• In Crossfire (not sure if this applies to SLI), memory clock for the primary GPU is ALWAYS on load IF YOU ARE USING MULTIPLE MONITORS. If I recall correctly, this was design by AMD to prevent a flickering issue from memory frequency to low to drive both monitors. This also results in increased temperatures on the primary GPU
• I got around this issue by forcing lower clocks with MSI Afterburner with unofficial overclocking settings for idle/2D mode. 
• Update 10/19/2014: This was fixed in more recent catalyst drivers (I think)
• Update: I added a dedicated Physx Card in conjunction to my GTX 670 (I got a cheapo GTX 465 for about $40 off ebay). Despite the fact that the cards are not in SLI, and are also well spaced apart, the GTX 670 runs about 10C hotter on load with the 465 in the case! I did not anticipate simply having the extra card would cause such a significant increase in Load temperatures. Of course this also maybe just because the dedicated (465) Physx card is Fermi architecture video card. Fermi runs too hot, and uses too much power for the performance it provides. (It is a Galaxy GTX 465 with a 'flip fan' design.)
• Video RAM does NOT Stack in SLI/CF
• A common misconception is that if I have one Video Card with 1 GB, if I get another one and put them in SLI or CF, I will have 2 GB! No! This is incorrect unfortunately. 
• This issue even applies to sandwich cards (dual GPU, one PCB): ARES, x2's, MARS, GTX 690/590, AMD 7990/6990 etc or those special cards (dual GPU, two PCB using with one PCI-E bus) GTX 295, Geforce 7950 GX2.
• Think of it like this: Pretend Video Games are like moving stuff from your old house to a new house. You have a choice of DUAL GPU--Two Toyota Corollas, or a single GPU equivalent--Ford F150 Pickup Truck. We'll say that video memory (lets call it VRAM) is the capacity to hold things in each car. So the corolla has a small trunk but has seats that can hold stuff. So let's say that each corolla is equivalent to 1 gigabyte of VRAM, and technically since I have two corollas, I can bring twice as many things in one go--I have technically 2GB. The Ford pickup truck is well..a pick up truck and has LOTS of carrying space, so lets say that the Ford on its own has 2GB of VRAM. 
• For the most part if I only need carrying many small objects, these two Corollas maybe be able to carry just as much things as the F150. In this case the games don't use much VRAM and as a result these dual GPU's can perform just as good if not better than the single F150.
• But what if the game needs MORE than 1GB of VRAM? What if the object I am trying to move is large, like a refrigerator? The F150 will have no issue carrying this. The two corollas on the other hand..well..tough luck. Technically they have 2GB but since these cars are physically independent of each other, they don't combine individual capacity into a singular usable net capacity. 
• In the same manner, the two graphics card in a Dual GPU setup are physically independent of each other. Each card can only use its own 1GB (or whatever amount of memory it has) to produce a frame. Then these two GPU's combine their frames together to produce an overall net increase in frame production.  
• Some video cards of same generation/model but different edition may have variations in VRAM capacity. When running in SLI/CF, if you have mismatched VRAM, the smaller capacity is used. A general rule of thumb in computing: you are only as fast as your weakest link.
• This also applies to HYBRID Crossfire, the faster card will slow down to match the performance of the slower card.
• Windowed Mode disables CF (and sometimes SLI)
• If you do not play games on fullscreen, multi-GPU setups will be problematic. This includes borderless windowed mode--yes that's right you alt-tabbers.
• CF Simply does not work in non-fullscreen, and I have confirmed through GPU-Z. Even If I force 3D clocks using Afterburner, during windowed mode or borderless windowed mode the second GPU will be at 0% utilization.
• I do not know the technicalities of why this occurs, hopefully I find something about it and will add it here.
• Some people claim SLI sometimes works/sometimes doesn't with Windowed Mode. I never checked this when I was running 9800GT's and those are long gone, plus SLI has changed so I can't confirm with a modern pair of cards (perhaps if I SLI 670's? c:)
• Microstutter
• Microstutter is a visual stuttering effect caused by variations in the time it takes to render each frame, despite apparent constant high framerate.

Frame rate is not measured at Hardware (GPU) Render Time



• Is Microstutter real? Yup. Some people on forums claim its placebo but it's real as I have experienced first hand. However, sometimes it is more difficult to see. For example one of the demonstrations I was shown was in the video game Sleeping Dogs. In that video, which was slowed down, it was blatantly obvious that there was stutter--BUT, prior to seeing this video I had I played through the entire game on my 6850's CF @ 1080p maxed settings and didn't notice any stutter at all. My first experience with microstutter was actually in Far Cry 3. When running through the jungles with lush flora, it was very clear that my game was stuttering despite a purported 60+FPS measured by Afterburner. 
• This also propagates an issue with BRAND Choice.
• NVIDIA's SLI was claimed to produce 5% frame variation in a single GPU setup vs 7% frame variation in a DUAL GPU setup (GTX 680s).
• AMD's 7970 single GPU test had a variation of 7% compared to a staggering 85% frame variation in Crossfire X.
• Source: SWEClockers Article (in Swedish)
• "AMD could meet (and beat) NVIDIA on frame rates, only to lose out on smoothness as a result of poor frame pacing," Ryan Smith from AnandTech.
• On the other hand, AMD has recently released a Frame Pacing driver to reduce this stutter. This is a software solution that looks at variation in frame time and introduces artificial delays to smooth out this variation. Of course this results in overhead, roughly 5% performance loss: HardOCP Article
• However it has issues with with EYEFINITY and 4K Resolutions
• Source: Techreport 13.9 Article
• IIRC, NVIDIA uses a hardware solution for frame pacing, as a result has has a more consistent frame interval without the same kind of overhead.
• Generally Crossfire scales better than SLI but is not as smooth from frame variation.
• Problematic with Video Capture
• With Software like OpenBroadcaster, FRAPS, DXTORY, X-Split, multi-gpu setup is problematic. However you can bypass this problem with capture cards--use a secondary computer to capture video output and record/stream from that computer.
• Software like OpenBroadcaster pull the frames directly from the primary GPU and does not grab frames from the video output. As a result despite having multiple-gpus producing frames to create a unified smooth experience, the software only pulls the frames produced on the primary GPU. As a result, despite a purported high frame rate, the actual recording/stream will appear as if it is stuttering if you rely on both GPU's to produce a unified smooth experience (ie: If both GPU's work together to produce 40 fps, the recording will only be about 20 fps produced by the primary GPU).
• OBS Support for CF/SLI in the future, not looking good. 
• From my own experience, however, OBS wasn't "pulling frames" off the primary GPU. Instead, Crossfire would work right until the point I hit "Start Recording,"--then the GPU#2 utilization percentage would drop to 0. The game I tested this with was Hitman Absolution. The OBSProject forums has numerous threads like this describing the issue.
• Some people claim this issue is GAME DEPENDENT but I feel that since the issue revolves around the the design of SLI/CF, then it should be an issue that persists regardless of the application. 
• I think the people who claim that OBS works fine with SLI/CF probably aren't even checking if the second GPU utilization greater than 0% while recording
• Or rather don't notice a framerate drop significant enough to warrant a check with CF/SLI being disabled  (as they have very high end cards, or recording a non graphic intensive game-- card is able to maintain fluid frame rate with one card [30-45+ FPS])
• They maybe even attributing the framerate drop as nature of recording software (framerate drops are normal when recording but the amount I was experiencing virtually made the game unplayable)
• Despite streaming numerous times before, I didn't even notice this issue until the last few months that of using 6850's in Crossfire--I attempted to stream Hitman Absolution at near max settings 1080p, when the game would choke horribly while recording, but play very smoothly when OBS was not recording). The game was running at 1080p but OBS was recording at 720p.
• Off topic: I am by no means a big streamer, but I stream privately to my small group of friends to share with them what I am doing. At times we even stream ourselves doing work or studying/homework! Basically its screen-sharing without Skype or Google Hangouts. 
• Personally when testing Video Capture with Crossfire X, DXTORY is the only application that supported multiple GPU video capture but it was seriously buggy and unstable (crashes, freezing etc.) OpenBroadcaster--Crossfire works right until you press that record button, then you can see the second GPU will drop to 0% utilization.
• Overclocking limited by weakest link
• While overclocking is possible with a multi-GPU setup, you are limited by the card that overclocks the worst. This is due to the nature of Crossfire/SLI which syncs the cards to perform at the speed of the slower card.
• Higher Cost to Upgrade, Dead End Upgrading
• For those who stuck to the single GPU route, they now have the capacity to tack on a second GPU to improve performance. If you used 2 or more GPU's for your current setup but you still want more, looks like you will need to scrap them all. 
• More Noise
• More fans -> More noise

• Greater Price to Performance Ratio
• Although the negative intricacies of Crossfire/SLI may seem to stack heavily against the number of benefits, the VALUE of a product should always have the highest priority making deciding on a purchase.
• Crossfire/SLI provides a really solid value for its cost:
• For games that do support multi-gpu configurations, 6850's in Crossfire perform just as well as GTX 580's
• Retail cost of a GTX 580 is $400+, retail values of of each 6850 was about $150. That's nearly $100 less for similar performance. 
• In actual cost, I purchased my 6850's brand new for ~100 each when they were nearing EOL.
• 6850 Crossfire Benchmarks
• More 6850 Crossfire Benchmarks
• As can be seen in the benchmarks, adding a second card in crossfire effectively doubled the frame rate.
• Greater Performance Capabilities
• For those looking for just raw performance, building the fastest of the fast gaming computer at any cost, you will need to go the route of multi-GPU.
• KingPin sitting at the top of 3DMark with 4xTitans
• Better Crypto Currency Mining
• I don't know much about this as I personally have never mined Bitcoin, LiteCoin or even DogeCoin, but most mining setup's I've seen use multiple GPU's running underclocked since it you want to have a net gain from mining after considering the cost of electricity. Multiple underclocked GPUs appear to provide better efficiency over a faster single GPU setup.
• Personally this is not a benefit for me but it maybe to others.
• This propagates an Issue with BRAND CHOICE:
• AMD cards are more efficient and overall better at mining crypto currency. I've read somewhere claiming that the reason for this is due to AMD's GPU architecture--it supports a bit shifting instruction which (due to the nature of cryptocurrency) increases efficiency of mining by a substantial amount (this instruction has little benefit to games). As a result, AMD's card prices are currently inflated (1/7/2014).
• E-Peen (Internet Ego) Boosting
• Two is better than one and I have two! Haha! I'm better than you cause I have more!
• "More is better"

Doing a Case by Case Look:

TL;DR: Use this beautiful chart I made (Just kidding it needs work)
Open the image in a new browser tab as it is too large for the image viewer on blogger...

Reasoning behind the Chart:

My current Mobo choice doesn't support CF/SLI!

If want to upgrade and your current motherboard doesn't support SLI/CF then you should stick with the single GPU route unless there are some goodies in addition to just CF/SLI for getting a more expensive board. If you are happy with what you have, why spend MUCH more (cost of mobo + cost of GPU set) just to try out something you may not even fully appreciate?

Is it worth it if I change my mobo (to support SLI/CF) ?

It's really up to you if you want to spend a bit more. Personally I feel that most people get higher end boards for more reasons that just SLI/CF. More expensive boards provide capabilities like more extra power phases for CPU OCing, greater number of 6Gbps SATA ports, better onboard audio, onboard SATA controller, higher memory clock frequency support and of course E-peen. 

Keep in mind though, higher end boards do have greater power consumption (10-20watts). This is generally nothing to most computer builders but IS reduced inefficiency nevertheless. If every watt matters (you are paying for electricity, tight on money...take a step back for a moment)

• Motherboard Power Consumption Variation

My power supply is inadequate or less than 650Watts!

I actually used a 600 watt Power Supply for my 6850's in crossfire--and this was in conjunction with an ASUS P67PRO motherboard, Core I5 Intel 2500K, four hard drives, 4 x 4GB Patriot Viper 3 Series RAM sticks, and 8 case fans. Check the  wattage of the cards you want to SLI/CF. If you are looking for a low wattage rig with less than 600 watts, you should probably avoid the multi-GPU route. If you need to spend more on a higher wattage power supply in addition to the costs of the GPU's, you may want to take another step back.

Consider the multi-GPU route: There are two good routes for dual GPU setups, Mid-High Multi-GPU or Flagship/Enthusiast Multi-GPU. Do not bother with SLI/CF if you are planning to use low end GPUs, the value is nowhere near the mid-high+ range multi-gpu setups. Back in high school I had a friend who rocked 9400GT's in SLI....9400GTs in SLI produce half the frame rate of a SINGLE 9800GT (9800GT's were ~$100ish compared to 9400GTs ~$40-50 each).

The reason I also feel Mid-High Multi-GPU's are a sweet spot because not only are they good for the price to performance ratio, but when you play a game that doesn't support CF/SLI, since the single card is already a mid-high ranged card, so it doesn't sting as bad when it doesn't work (MID-HIGH Ranged cards refer to cards ~$250 or above in costs, such as the GTX 760)--while the GTX 760 is no where as fast or expensive as the GTX 780TI ($700?), it can definitely play most modern games at 1080p at max or near maxed settings at about ~60 frames per second. So say if you put two GTX 760's in SLI, even though if you play a game doesn't support SLI, you'd still have an reasonably enjoyable experience. 

Type of Gamer/User is IMPORTANT

As I have pointed out early, there are a lot of intricacies with using Crossfire/Multi-GPU setups. For Multi-GPU's, the individual needs be someone willingly to work with their computer and understand it may take a bit of effort and tweaking to get the best performance. If you just want to buy a card, install drivers and expect the best performance you can get with that card instantly, don't bother with Multi-GPU.

The Gamers:

The person who is a 'hardcore gamer' or potential 'pro gamer' that plays that one game and that one game ONLY.
Well then does that one game support Crossfire/SLI?

The person play triple A game titles, hasn't touched an indie game before.
Well then lucky you, most big name game studios use big name game engines like Unreal, EPIC, Unity, Source Engine, for their games--and of course they support multi-gpu configurations out of the box. As a result you can pretty much guarantee triple A titles will support Crossfire/SLI (but don't hold your breath, some dev studios just want to watch the world burn). Blizzard is one of the exceptions as I've always had negative scaling with Blizzard games on multi-GPU setups. However more recent benchmarks it seems like World of Warcraft likes SLI (but not Crossfire), Starcraft benefit very little from multi-gpu setups. Nevertheless even most in-house engines by big developers like Frostbite by DICE support multi-gpu setups so overall if you play primarily big studio games, it is pretty safe to assume that SLI/CF will work.

The person who plays mostly Indie or Small Developer/Free to Play type games.
I do not recommend going Multi-GPU for this case. Most indie games such as MINECRAFT do not support Multi-GPU configurations and probably never will. There are some exceptions, for example the indie developers that created Shadowrun Returns (Harebrained Schemes), used Unity Engine. The game, despite being developed by an Indie studio supported Crossfire since the engine supports multi-gpu setups out of the box. 

The well rounded gamer; the one that does not discriminate and plays them all like Pokemon.
Some will work, some will not work (obviously) which is why I recommend mid-high or flagship GPU setups so the games that don't support multi-GPU setups still play well (just probably not at maxed settings).

The gamer but has workstation necessities
This one is hard to say. I personally have never had any benefits from using a multi-gpu setup for non-game/benchmark software and had issues getting to them to work for video editing software (After Effects), or stuff like Photoshop. Then again my GPUs are not workstation cards but game oriented video cards. Since the new Mac Pro has two workstation GPU's it must be beneficial right?