The biggest game in history, delivers on every front.
The online is where Halo 3 absolutely shines. There are all the match types from Halo 3 and a few knew ones as well. There is up to four player Co-op online, which runs extremely smoothley, and I have had no problems. The co-op is best played on Legendary, all others are way too easy for four players to do. The competetive online is amazing, you can search for certain matches, and those are found very quickly and matches you to players who are closest to your skill level. All in all Halo 3 is an amazing game. While the single player campaign might feel weak to non Halo players, I think most Halo fans will not be dissapointed. Even if you don't expect to enjoy the single player, then simply go online and enjoy the greatness of Halo against or with other players.