It is it was an expansion and I agree with the guy that it was so short it could possibly have been a download from the XBL marketplace and for much less. The only people who really benefited from buying ODST were people who didn't have any of the halo 3 DLC but even then you still bought something you already had if you owned halo 3 which was the multiplayer. They could have just bundled all the halo 3 DLC onto the second disc instead of a whole copy of the MP game and and gave it a lower cost I would think.
I was hoping ODST would have been good but it was pretty boring and the story just wasn't there it didn't even have the same feel to it it just seemed very pointless and I was rather disappointed . The graphics for some reason in ODST seem like they took a step down from halo 3 as well it was weird but going back and playing halo 3 everything was much more flush compared to ODST. All I can say is if there's another expansion it better be worth it the next time and I'm really going to be expecting allot from reach after this and if reach doesn't make up for something I probably won't look twice the next time I hear about halo.