I'm echoing Drinky's complaints. I too have an uber-rig, and the game performs so unevenly. It goes from nice and smooth at 1680x1050, maxed graphics, to choppy bitchcakes for just about no reason at all. Sometimes, the amount of action on screen isn't enough to warrant slowdown.
Also, it's day 1, and the game already had an 80 meg patch.
The game seems good, but the performance issues are befuddling. They need to be patched fast because it's going to be annoying to play as it is.