if you only owned one game on Xbox 360, Halo 3 should be it!
The biggest innovation is this game is the organization for setting up an online match. The game allows you to set the options or just jump right in. It also allows you to have more than one person playing on your system, However they must each have their own gold membership, to compete against people from all over the world. Finally, a game that you can play with a friend who isn't playing on their own xbox miles away at their own house!
The game itself plays very well. The loading is done while you are playing which prevents frequent pauses for loading, much appreciated. Also, the framerate is very consistent, even with all the multiplayer options. The story is not extremely long (it will take several hours to get through), but you'll be playing through it with friends, so it allows you to play through the hole thing in one marathon session. Replay value on the campaign is also high.
If you play Halo 3 by yourself, you might wonder what all the excitement is about, but given the ease setting up multiplayer, and the amount of options there are, I doubt anyone will be playing by themselves much. In summary the game plays extremely well and is a must own for anyone who has an Xbox 360. There is a lot to do and I plan on spending a lot of time playing this game in the future.
Edit-
After playing several more times it no longer allows me to play online with anyone who has a silver profile. I've corrected the main article to reflect that. You can still play two people online on one system, which is still new. However it is not as good as being able to play online with only one xbox gold account. I have adjusted my score to reflect that.