Multiplayer is amazing, single player gameplay: awesome, but something's missing...
I would have given this game a 10, if it wasnt for one thing (which may not bother some players). The storyline. It just wasnt imaginative. This was supposed to be the final chapter of halo, but it still feels like we're waiting for some closure. A Halo 4 would not seem out of the question after seeing the ending of Halo 3.
Although the ending may not have been quite as bad as in Halo 2, it still gave me the same feeling of "well...then what?".
Other than that, the game is amazing, both the single player and multiplayer level design is great; and even if some of the cool weapons have become slightly less cool (no more heat seaking rocket launcher *cry*), even cooler weapons have made up for it.
Certainly the best FPS on any console. You wont feel like you wasted your money if you buy it. This game is worth it even if you only ever play the multiplayer.