Its exactly what you expect if you are a fan of the series, but otherwise offers nothing new.
Campaign
Although Halo 3 has great gameplay, its campaign is actually quite bland and boring. Its story is hardly immersive, and you dont really feel how Master Chief feels. Also, the story feels sort of thrown together and it really does not make sense from the players point of view as it looks like the player is thrown through levels which dont really have much to do with the story. Another complaint about the campaign is the repetitiveness of the Flood. That was plain boring and annoying. It just was not needed.
Multiplayer
Plenty to do here. Halo has and still does offer some of the best multiplayer experiences. Ranked and Social playlists in matchmaking are available for the hardcore gamers and casual gamers. Maps will be available for purchase as well. The maps that come inside the box are also incredible except for a select few. There are few glitches in the game and the gameplay is sometimes hampered by lag. Map design is also great, with much terrain and objects.
Extras
The addition of the equipment function is great but it does offer some cheap tactics in multiplayer. The theater mode is incredible as it saves both campaign and multiplayer clips for you to watch.