Halo 3 is a must-buy for any 360 owner.
Halo 3 in it's very glory is a highly addicting and fun game. The campaign consists of 10 (or so) levels, each is unique and design brilliantly. You can play alone, or with three other people, either locally or online. Ofcourse, it's more fun to play with other people. In campaign, you get to play as Master Chief, and you basicly shoot aliens. But it's really, really, really fun. The only thing I can really complain about is the horrible weapons design, you don't even know what the weapon does before you try it out. The multiplayer is also very impressive. There are two main multiplayer modes - Custom games, where you can play on custom maps and also pre-included ones, create custom game types etc. Then there's matchmaking, which is basicly you and your party going out and blasting the **** out of everyone (or the complete opposite).
All in all, Halo 3 is an awesome game which is a must-buy for every Xbox 360 owner.
Confusing weapons design.
In matchmaking, it's not that easy to get to a game type/map that you like.