Game's fun. It's the where Rockstar would've gone after San Andreas had they not decided to do a more serious take. The automatic pistol you get from the Fuzz missions is fucking awesome. Pimping out your car (and by pimping out, I mean adding spikes to the wheels that shred other driver's tires) and then saving it in your garage is nice. The fact that if you lose it or get it destroyed you can just buy it back is fantastic. That and cruise control now need to be staples in sandbox games like this.
The only real complaint I have, and this just in response to some other stuff people have said, is that the story is crap and bullshit. It's kinda fun and decently acted, but wavers between serious and silly too much. Like the secret mission where you find, fight with, then kill Keith David from the first game. The lead's reasons behind it just angers me so much, that bullshit gang machismo. I felt like an asshole after that. The lead character is such a fucking small minded dick that it is stunningly difficult to sympathize with her/him/it. It's like you're playing as CJ's brother from San Andreas, you know, Mr. Grove Street Fo' Life, the one everyone hated cause he was so, well, small minded and dense. It's the difference between characters that makes GTA IV the far superior game, in my opinion. Not that I haven't been having loads of fun. Just mostly when it doesn't involve story bits. Like the Hitman missions, which are way better than GTA IV's.