It's good and you should buy it. No question.
I've never been a fan of the GTA franchise. Ever. I got sucked up into the hype of GTA IV, however, and sort of just went along for the ride.
I picked the game up on launch day and upon tossing it into the Xbox 360, I was immediately bombarded with game invites. I wasn't even allowed to watch the opening intro or credits. I joined a group (10-14 people) of LoadingReality.com folks and started off in a Cops 'n Crooks battle.
It was a blast. In my first two hours of play, there were at least five "holy crap" moments. Here's one that I remember:
Uncle Chase driving a pick-up truck straight at me, without wavering. I took my AK47 and popped two bursts into the driver's head, without locking on. He died and the truck kept coming. As gravity would have it, trucks don't stop moving just because the driver dies. I got squished.
I've spent about 15 hours or so in-game, and it's been nothing short of "very good." I can see the possibility of boredom from the game, but am impressed by the vast amount of things to do that keep it fresh.
I'll be playing this game for a while.
The "good" section above was written 15 hours into the game. I haven't played it since that point, due to the level of boredom that set in after the flair of multiplayer ran out. Fans of the franchise will love it, but those of us that are used to non-stop action (a la COD4) will be left wondering what's next.
The Bottom Line
Presentation: Nice. The game is laid out as good as it can be, the UI is flawless, and the cell phone menu controls are a nice touch for immersion.
Gameplay: Nicer. The cover system is really well-implemented and the combat itself works nicely. Driving is realistic and once you become a master of the handbrake turn, you're in there like swim-wear.
Graphics/Sound: Nice. The graphics are decently strong and push the hardware quite a bit. I noticed a bit of popping and vehicle glitching, but only about 30 minutes into huge free play, multiplayer games. The audio is as good as any game, ever. It's one of the things that the franchise is know for. The radio is so good that it hurts.
Value Factor: Nicest. The multiplayer is surprisingly strong and when combined with a true open-world experience, there is incredible freedom and replayability in the game.