This is Grand Theft Auto and that's all you need to know. Play it.
This is actually more role-playing game than some would like to admit. Granted, you don't "level-up", but there are many ways in which you can or need to improve your character to do things or to do things easier.
Some might say the game world is too large, but I think it is just right. It's really not that big of a chore to go from city to countryside if you need to. If you complain about the world being to big then play Morrowind and get back to me on that one.
The sound of the game is great, the music is even better. I particularely liked the soundtrack cause that is the era in which I grew up in. Hearing Cypress Hill, Gang Starr, Masta Ace, Public Enemy, Ice Cube, Helmet, Eric B & Rakim etc was just way to awsome. I didn't expect nearly anything as sweet as the soundtrack that they did come up with, even though I knew it'd probably be good. Sure the soundtrack for Vice City was good, but it was more gimicky than this soundtrack, this one is just more listenable on many more levels.
Graphics are about as good as they can be. Jaggies on the xBox seem to be worse than on the PS2. Also, other than draw distances, I think the graphics looked smoother on the PS2. But overall it's really a wash.
Controls: You'll hate flying and drive-bys on the xBox, especially if you've played the ps2 version. Yes it is that bad. I really think they should have given the option to customize the controls for this game because frankly, most gamers could have layed them out better.
If you have both a PS2 and an xBox, buy the PS2 version of the game. I am serious about this. The controls factor makes that much of a difference.
"But I won't get the 30 second replays!" you might say. "Bah!" I'll say to that, cause you can't save them anyways. "But the loading is faster on the xBox!" you might say. "Bah!" I'll say to that cause it's such a small difference. "But the textures are better on the xBox!" you might say. "Bah!" I'll say to that, you can't even tell a difference unless you have two high-res tv's right next to each other, plus jaggies are worse on the xBox.
I've played it many hours on both systems, trust me.