I'm using 2 GTX560Ti's in SLI and from one days experience I've found:
Civilization 5 has loads of flashy textures when SLI is enabled - really horrible.
BF3 actually has big frame drops at times when I'd never get them (mostly on the new maps).
I haven't tried Skyrim and MW3 since then (supposed to have improved a lot according to Nvidia) but I might be rolling back very soon.