Got sucked into GTA IV this weekend. The physics are a lot of fun. So far my only gripe is that the sniper zoom is broken on the PC - remapping doesn't work. I've made it through the first 2 mission with sniping without zoom.
Performance wise, it might be CPU bound. I'm running a triple core @ 3.1ghz and the difference between 1600x900 and 1920x1080 is around 2-3fps. But at 1080 the game really chugs at odd points, usually indoors like leaving the apartment where you wouldn't expect it. As soon as I get outside the frame rate picks back up.