From a technical standpoint, this is the best GTA game yet. With a bazillion side quests and things you can do, it can keep you busy for many, many hours. The best added feature is the two player rampage mode. Suit up with a bunch of guns, then find the two player spot. Then run around, one person as gunner, the other as a driver, or whatever. Nearly anything you want to do in this game, you can. As long as what you want to do involves mayhem and death.
Storywise, eh, it's ok. Maybe it's just me, but half the time I'm not sure what I'm supposed to be doing because I don't understand what the hell they are saying. I must be too far removed from the Gangsta scene. But it's fun, and has it's moments. If you liked the previous games, I doubt you'll be disappointed. If you thought they were a travesty to mankind, well, this one probably is too.