Bought it at launch and gave my opinions about it in an older thread.
Basically what I had problems with were the crummy compressed videos and some crashing early on. I hear the game is much smoother than the Xbox version, though I wouldn't know for sure how much. The aiming is mouse aiming now, so the complaints I hear about flick targeting should be near nil unless you use a controller. It never slowed down for me, but I have a high end system. It should run pretty well on medium end systems, the graphics aren't dazzling in any way.
Fun game, I reccommend it; short and easy though. If you could be more specific, I could answer more questions.