This is what I experience:
First load the game lags like crazy. Levels out after about 5-8 seconds. Then frame rate is smooth, but will drop off if happen to be looking at the wrong thing (what the wrong thing is I don't know it will just drop noticeably when moving around at times). When I first fire a gun, the game pauses. Then seems to work normally after. Opening the inventory creates a large lag spike at first, then its really jerky moving around inside it. But it will go smoothly, but them seemingly start missing mouse input for a second. There are many times during gameplay when there will be micro stutters. The micro stutters seem more prevalent in enemy battles.
All in all it grates and I just found myself 'waiting' for the next stutter instead of playing the game. I have been playing Diablo 3 and StarCraft 2 quite happily.
I have been running at 1440 x 900 in all my tests btw. I tried 1024 x 768 but geez that is low. I mean it does not even take up the full size of the screen....performance seemed improved but the issues above did not vanish fully.
Thank you for this Rob, feedback like this enables us to make games better. I'm going to PM you so we can dig in here and hopefully find you a solution. I switched over to 1440x900 and while I did notice an increase of load-in lag (especially when loading into Sactuary, where a bunch of stuff is being rendered), the mini hitches has me worried. That should definitely not be happening, and we haven't seen it here in the office.
We play Diablo 3 and SCII all the time at lunch