Finish the fight all too quickly..
The campaign for Halo 3 is very short, you can knock it over in a day. Gameplay, both on and offline, are basically 90% the same as Halo 2, so if you like Halo 2, chances are you will like Halo 3. The graphics often get criticised in this game but I think they are great, definitely a big step up from the last 2 Halo games.
The Forge feature is a great idea, as is the movie viewer, I hope these 2 features find their way into many more games from now on.