One of the best online games!
Well, that goes for the game. Community on the other hand, is one of the, if not THE worst community I've seen in a FPS game to date. There are tons of whiny little kids, rude people, racists etc. It takes away a little of the fun, but I wont punish the score of the game because of that, since it is not bungie's fault, and any major game gets a community like this. Nothing the devs can do about it.
The campaign is "alright" not the best I've played, but hey, the game is for online, and there it delivers, but it is also a good campaign ending for those who have been following the Halo Trilogy.
All in all Halo 3 is a hit, which sales show, and should be a definitive buy for any shooter fan, and everyone who has a xbox 360.