ODST is really just a side story. I don't think anything was too important in the grand scheme of the Halo story line. From a game playing perspective, since you are an ODST you don't have Spartan strength and ability and this really effects how you handle situations. Hunters for example are a much bigger pain in the balls, lol. I'm sure you can get the game dirt cheap by now and I'd say get it even though I wasn't as enamored with the game as a lot of other people were/are.
The Halo 3 in the title probably has more to do with the multiplayer map packs that came with the game.
**Edit** Yeah Dirty beat me to it :)
I LOVE paying for Xbox Live! I also love that my love for it pisses off so many people.







