F**k the single-player (although the storyline is great), it's the multi-player on Xbox Live is where it comes into its own.
There are game modes for everyone:
Slayer, kill anything that moves.
Team Slayer, work together and kill the other team.
CTF(Capture the Flag), team based attack and defend.
And there are even some stupidly fun ones like Pirate-Sharks.
There are also a great deal of weapons which are excellently balanced.