I missed out on ODST even though I've long since played Halo 3 and Halo: Reach.
After spending a bit of time with it, I've quite enjoyed the single player campaign. The story of the game is a side-story set around the time of Halo 3. You never meet up with Master Chief, or the Arbiter so forget about them. You'll be in control of members of the ODST and you'll get to see the war from their viewpoints.
Granted the single-player is very short, and some of it like wandering around the city to find clues is more filler than anything. However it has plenty of moments, and once the real missions start they're pretty good.
The unfortunate thing about ODST is that there really is no multiplayer to it. They throw in a bonus disc with all the Halo 3 maps, so if you haven't bought those this is a steal. However if you did, then half the experience is ruined. The game has dropped pretty well in price though, making it a very affordable purchase than the questionable price tag of $60 on day one.
Score: 7 out of 10