This expansion turned retail game is good, but fails to fill Master Chief's shoes. More spin-off than expansion.
Firstly, there is no Halo in this game, and only a few passing references to the events of the trilogy's story. Secondly, the entire game takes place in a 24 hour period that takes place in the first 1/3 of Halo 2. I feel ODST would have been better served as a spin-off game taking place in the same universe instead of trying to shoehorn it into the Halo game series. But nitpicking aside, let's continue with the pros and cons of this latest in the Halo series.
Throughout the game, you play as each member of your ODST squad. Though the voiceless Rookie is your "main" character. You wake up 6 hours after a botched orbital drop in the night-dark city of New Mombasa alone and without backup. Jazzy film-noir music plays as you explore the deserted city looking for your squad. Occasional skirmishes with Covenant forces liven things up. Each time you find a clue to the whereabouts of a squad mate, you get a flashback mission where you play that person. These flashback missions are much more like the missions and battles you fought as Master Chief in the Halo games.
Eventually, morning dawns and you get your climactic battle, complete with daring escape from the city just as the second wave of Covenant ships arrives to excavate the artifact which is depicted in the beginning of Halo 3. Throughout the night leading to this point, the city's enigmatic AI has been feeding you clues to a secondary story, reminiscent of an old radio play. Between the audio clues, and the final couple missions of the main game, you are rewarded with insight as to how humans prevailed by the end of Halo 3.
I believe there are some continuity flaws with the story. In Halo 2, when the prophet's carrier slipspaced out of New Mombasa and the Chief followed, the city was supposed to be pretty well destroyed. Yet as you explore, you find the city is deserted but mostly intact. In the Halo games, the Master Chief speaks. When you play as your squad mates, they speak. So why doesn't the Rookie ever talk? And while the concept of flashbacks, the film-noir aspects, and the perspective of an ODST squad is all well and good, I felt the game seemed slightly disjointed. I think by branding it as a Halo game, I was expecting something more… Halo-like. Bungie should have just named it "ODST" and left it at that. Then I wouldn't have Halo on the mind so much.
The Firefight component of the game is a wave based "defend this area" mode. Best played with friends. I had a flashback playing this to the old days of Unreal Tournament. The announcer stating what skulls are in effect, calling the wave numbers, the number of enemies left, and the number of lives left all reminded me of that "sport" feel. Plus you have multipliers, sprees, and other announcer friendly callouts. The mode can be a lot of fun, if you are playing with friends who work well as a team. And lastly, you have the multiplayer. There's really not a lot to add here, other than the fact that it is pretty much a wholesale copy and paste of the Halo 3 multiplayer including map packs.
Overall, Halo 3: ODST is a good game. Though I have to admit that it might not quite be worth the $60 price. Perhaps a $40 budget-pricepoint would have suited the game better.