ubercake :
I don't experience microstutter with my two 680s in SLI. I had it constantly with my 2 5850s and still with 3 5850s. I never had a problem with microstutter with 2 580s in SLI either.
I challenge everyone to try both multi-card technologies for themselves and then talk about their results. Don't just take the word of others as the truth.
You can't use both CF and SLI and get useful results from just that context. Different GPUs (especially of different architectures) can vary in effectiveness with the technologies even with the same drivers (granted the drivers are a part of it). Radeon 5000 cards and the other graphics adapters that have GPUs that are VLIW5 are all known to have had above-average stutter issues in Crossfire compared to a VLIW4 or GCN GPU of a similarly performing graphics card. For example, this is why Radeon 6850 Crossfire is usually said to be much more stutter-prone than Radeon 7770 Crossfire regardless of the drivers being the same or different no matter what version they are. This is despite the two cards having comparable performance in single GPU situations.
Also, faster GPUs tend to reduce stutter compared to slower ones. For example, this is why GTX 560 SLI is usually reported to be much more stutter-prone than GTX 580 SLI. Yes, Nvidia does and has had issues with this as well. All graphics cards, especially dual-GPU graphics (be they on a single card or on two cards), fail to have perfectly consistent frames. A single GPU card usually keeps them far below any perceptible level, but not always (regardless of the GPU's designer). Dual-GPU setups pretty much always make this issue worse, but not always so much so that it is perceptible by most people. Extremely high end cards such as the GTX 680 rarely have significant issues, but that is not a guarantee to never have issues in any situation.
The drivers are also a factor. For example, Catalyst 13.1 is usually reported as smoother than previous versions and Catalyst 12.7 is usually reported to be smoother than previous versions. AMD is also not the only one that has had driver issues with this even recently. For example, for a few months after their launch, Nvidia's new drivers had some issues with stuttering. It might not have been micro-stutter, but instead a different form of stutter, but it was stutter nonetheless. Another aspect of drivers is that not everyone has the same issues even with the same hardware and driver version. So, not everyone had this issue. However, there was a large number of people (large enough that Nvidia had to recognize it and a few other issues and make a statement about fixing it) with the issue.
These are the main graphics-related factors. Software/operating system, other hardware, and much more can also have an impact. Except for significant bottle-necks such as pairing a GTX 680 with an Athlon 64 x2 CPU or some other huge bottle-neck such as using extremely slow RAM, the rest of the system is usually not of huge significance. However, it's still a factor.
My point is that if you want to make a comparison for micro-stutter, you have to take in all of the factors and make several apples to apples comparisons to isolate many of these factors.