Halo 3 ODST tries to stand by Halo 3 as an equal, but comes off as more of a cash-in than a strong expansion of Halo 3.
The answer: Not perfectly well. (This is not including the second disc, since that's nothing new.)
Halo 3 ODST's campaign follows a group of ODST soldiers on Earth. The characters are cardboard cut-outs. The a-hole, the sarcastic weakling who has a thing for the only girl in his squad, the mute, etc... You are supposed to drop onto a Covenant carrier, but stuff goes down and you crash to Earth and get seperated.
To progress the story, you need to move the Rookie (the aforementioned mute) from city block to city block to start flashbacks, these flashbacks take you into the eyes of the other squad members. Although the characters are so interchangeable you can be playing as one or the other and not notice if you didn't see the character's name at the flashback's start.
The first thing you will notice is that you are not Master Chief. If you play through the game on Heroic and try to run balls-out into a group of jackals and grunts your hanging balls will get stomped on. This, along with the stupid grenade throwing will take some getting used to. This game is hard, and hard in a strange way. In easy sections, you'll have more health packs than you can even handle, but in the hard battles (which there are a few of) you'll have barely any.
I don't have a lot to say because not a whole lot has changed. Or rather, what has changed kinda sucks. For example, that grenade throwing I mentioned. You throw grenades higher than the reticle and higher than Master Chief. Meaning trying to throw a grenade right under a doorway to kill a group of grunts will send it flying towards your face like a super bounce ball because it ricochets off of the doorway above.
The almost useless map is another pointless add-on, especially considering the game is essentially linear. Once in a while you can see enemies on it, making it a giant inconvenient form of radar.
Firefight also demonstrates the lack of innovation. Bungie supposedly reused the Halo 3 campaign co-op code for Firefight. Which means no matchmaking for one (which actually isn't a big deal, three random players wouldn't really help you much I wager.) but my problem is no options! Where the no achievement-no leaderboard custom game mode? All grunts with fuel rod cannons, Cowbell on and 300% speed? Seriously, if only Firefight had the options of Halo 3's custom games, it would be an amazing gamemode. As it is now, it's something to screw around with when you're bored with Grifball.
The game is good and all, it's just...less amazing than was hyped. Everyone came their pants when this was first announced and to get something less than par is a little bit of a cock-slap to the people who waited months for it.
Eight of ten, play it, but don't expect it to break boundaries and push respective envelopes because you might be disappointed.