My X2 did fine in most situations. I noticed stutter and wild FPS fluctuations in large scale real time combat. It made raiding in some MMOs down right painful. I don't get that with my overclocked GTX 260.
[H]ard talked about the wild FPS variations and stutter @ high res in Clear Sky-
"Having faster average framerates does not necessarily always mean the video card will give you a better experience while playing the game..."
I'll be sticking with single GPU now and in the future, unless something drastically changes.