Great game, best in the franchise
Master Cheif is back, continuing the story left off from Halo 2, and the original Halo before that, in which the story is great, but I will spare you spoilers.
Multiplayer has to be the sole reason this game has a lot of replay value, not only can you play people by shooting their heads off, but they can join the fight and help you in co-op, and up to 4 people can join you.
Graphics are great, it may only be 720p but don't let this stop you from playing this game, as it has some very pretty visuals.
So if you have a 360 and you haven't picked up Halo 3, I highly suggest this game in your collection, if you have this game and looking to expand your replay value online, I suggest picking up the Legendary and Heroic map packs to get a feel of something new.