I played the main story and the biker's part too, but couldn't finish the pimp's thing.
I hated the game - badly optimized for the PC, poor graphics and the worst part of the GFWL.
Actually, after spending about 80 hours playing this game on PC, and also playing other open world games like Just Cause 2 and Saints Row The Third,in my opinion GTA4 is actually not that badly optimized. It's actually pretty well optimized relatively speaking.
The game has so much more dynamically going on than other open world games, you can't even really compare. With the level of physics, the size of the world (the city in GTA4 is MUCH larger than Saints Row The Third) the amount of detail in general, and most importantly, the euphoria physics engine. It really makes a huge difference.
Saints Row The Third is a cardboard cutout in comparison.