I have two way GTX 670 SLI and it is great. It scales very well in most games and I have never noticed any microstuttering. I play at 1080p with a 144Hz monitor, so I want an FPS of around 120 while keeping the graphics as high as I can. This is more demanding than 1440p at 60Hz I believe and I'm noticing that my 670s are running out of grunt with some current games on ultra.
One important this to note, I would only ever SLI/crossfire the top two/three cards offered by a company, e.g GTX 770, 780 or 780ti. In almost every case it is much better to buy the best single card you can and then SLI it later. In my case, buying a 670 was much cheaper than a 680 and the performance difference was incredibly small, especially when the 670 was overclocked. The only time I'd recommend SLI/crossfire configurations of lower end cards is if you will never be able to afford one high end card. This is because two low/middle end cards will outperform one high end card in most cases.