The embargo was lifted on Sunday so reviews for Halo 3 have been pouring out. You can see a breakdown of them here at Metacritic, where it has an overall score of 96 out of 100.
They all seem to say the exact same thing. The campaign mode is wonderful, but also a little on the short side. Co-op, however, adds massive replay value to it. Online multiplayer is just about as perfect as you can get and will be played for years to come. Overall just a really great game.