Halo 3: ODST, recycles the gameplay of earlier Halo games, which is not necessarily a bad thing, while it brings a few new things to the table.
The most obvious addition to the series is Firefight, a cooperative mode that throws wave after wave of enemies at up to four players. This is insanely fun. As the game progresses, the enemies get harder to beat, while the game also turns on skulls, which are the Halo version of cheat codes, that make the game even harder. I could get bogged down in the minutiae of this mode, but I will just say that it is really fun and you should try it.
The next part of Halo 3: ODST is the campaign. The story opens with the rookie alone and completely surrounded in an occupied city called New Mombassa, hours after getting separated from your squad. As the rookie, you are tasked with finding out what happened while you were unconscious. As you progress, you find clues, which trigger flashbacks. These flashbacks allow you to play as other characters, which shows what happened to your team. These are fun, but the real star is New Mombassa itself. Most of the game is spent exploring the city at night. The tone the designers attempted to create was of a film noir. The lonely jazz music and frequently empty streets helped achieve that. I'm not sure if I would credit them with succeeding entirely, but they were able to create a pitch-perfect sense of isolation.
Some of you may be interested to know that Nathan Fillion, Alan Tudyk, Adam Balwin, and Tricia Helfer voice characters in this game. Yes the three nerdiest things I enjoy: Firefly, Battlestar Galactica, and video games, all got together and made a [disgusting metaphor deleted]. Now, for much of the game, you play as the rookie, the mute, faceless character with no personality. I assume that the goal was to put the player into a blank character, so they could blah blah blah. My issue is that Halo 3: ODST is full of interesting and underutilized characters, and much of the game is played as nobody interesting at all.
The most common complaint I have heard is that the game is too short. I probably spent about six or eight hours playing through the campaign, but I also spent a fair amount of time exploring. I think it would be possible to finish in as few as three or four hours.
I think that the campaign in Halo 3: ODST, though marred by some nagging problems, is the best campaign in the Halo franchise. The thoughtful inclusion of Halo 3's multiplayer was a nice value-add, but the real prize is Firefight. Incredibly fun, this mode is easily worth half the price of the game. Now many of the reviews that I have seen address the price of this game. I think that is an incredibly silly thing to do, but here goes my shot at it.
Firefight: Easily worth $15, possibly as much as $20
The Campaign: I'd say I got $20 worth of fun out of it
The Halo 3 Multiplayer Disc: The three new maps will probably go on sale in a month or two for $10
So there you have it, Halo 3: ODST is a steal at $60. Now, this sort of math is total bullshit, but I have seen it thrown around in other reviews of this game as if it matters. The only thing that matters is do you personally think this game is worth the money. I for one do.