"Not a must-have, but a nice add-on"4.0 starson by Ptto911
Pros: New "Firefight" mode allows for co-operative multiplayer play, Halo 3 multiplayer disc with all map packs and addons + 3 new maps
Cons: Short campaign
Summary: While it may not impress veteran Halo players, Halo 3: ODST gives more insight of the story of Halo 3, and brings a new arsenal of weapons and features to the campaign, such as the new co-operative mode "Firefight" where you can kill enemies and build your score.
ODST also comes with a Halo 3 multiplayer disc that includes every map released for Halo 3, plus 3 new maps. The disc has the options; Matchmaking (online play), Custom Games (online or offline play with maps and gametypes of your choice), Forge(online or offline object editor that gives you the ability to create your own map), and Theater (online or offline play of films of Halo 3 gameplay).
Overall, Halo 3: ODST is a great addon for your game collection, but don't expect an entire new Halo game.