Yeah it's pretty much an expansion. I didn't play online but I played some campaign and that firefight mode.
Campaign was so boring to me. There are spots of just walking around looking for clues and times where it's actually exciting. I stopped playing campaign like an hour in because the slow times are too slow.
The firefight thing is alright I guess. The waves are much too large in my opinion. The level was saying that I was supposed to be getting like 200,000 points (at least that's what I think they meant by par) and I was getting like 20,000. And it isn't like I was dying a whole bunch, this was with continued play of about 20 minutes. Stupid if you ask me.
I wish I would have played online. I really like the pistol and the suppressor on the smg, and the fact that you aren't jesus who can run and jump and do anything he wants. I have the feeling I'd like ODST online more than Halo 3 online.