ODST is a competent game, that feels a little like an expansion pack with a few extras sprinkled in. Those extras, such as firefight mode, and the mythic map pack, seem to only exist to justify ODSTs full game price tag. Without them I would have a hard time coughing up full price for Halo 3: ODST.
The title alone reminds you of what this game really is: a repurposing of Halo 3s game play and engine to tell a side story in the Halo universe. Set some time during Halo 2 and leading up to Halo 3, ODST serves to fill in the gaps between the two games. It tells the story of a unit of ODSTs (futuristic paratroopers) that get separated during a covert mission.
You begin the game as the Rookie, yet another silent protagonist. The Rookie has been knocked out for six hours, and when he (or she) awakens they find the city of New Mombassa (a setting Halo fans should be familiar with) in ruins and his team missing. This is the beginning of what sets ODST apart from other games in the Halo series. You play as the Rookie in an open, "hub area", tracking down clues as to where your comrades have gone. Each time you find a clue the game switches flashback where you play as another member of the missing ODST team.
These flashback sequences represent the meat of the story. The exposition and character development happens in well written scripted sequences. You'll recognize the voices of each of the characters if you are familiar with recent popular science fiction; many of the voice actors were mined from the defunct Fox show Firefly.
During these flashbacks is where you'll experience the more diverse gameplay offerings of ODST; but those who played previous Halos will find it old-hat. Each flashback represents a typical Halo style level, focusing on one type of gameplay. There's your typical on foot, pushing forward, type level, the Warthog level, the sniper rifle level, and so on. After a while you'll know what to expect, and there's nothing really that new to the gameplay formula. After a while you'll begin to expect the alleged twists and turns, and many dedicated Halo fans may feel cheated by the games big twist reveal.
That's not to say this is a bad thing. Halo 3 was a great game, and more of it is a good thing. The slight twists on gameplay may, at first, seem superficial. The big difference being that, like the first Halo, you must find health packs to restore your life gauge. I found occasionally scrambling for health packs a lot of fun, as it added a level of tension that seemed to go missing in Halo 2 and 3. That's not to say you don't have a recharging shield gauge, because you do, and this time they call it "Stamina". Unlike Master Chief's shield, stamina depletes quickly, and when you're out you'll begin to loose health. Getting out of fire by taking cover is the only way to recover your stamina, a dance many Halo fans are familiar with.
The game was also afforded some graphical tweaks. ODST is very colorful and full of dynamic environment effects. Ash and radioactive snow fall from the sky, amongst other things. The Rookie wonders new Mombassa at night and the city if full of beautiful colored lighting. Police car flashers, and red emergency lights color the city in a unique glow.
Not that you'll see it much, at night you have to rely on your VISR, a night-vision type mode that highlights everything with different colored outlines and washes everything in a green filter. Don't count on using it in the flashbacks, however, as they almost all take place during the day, which renders the VISR useless.
It's worth noting that one thing stood out as one of the ugliest things to appear in a Bungie game. The female character Dare features one of the most hideous character models ever. Without a doubt. I'm not certain if Bungie intended for her to be ugly, but she sure is a minger. When standing next to Buck, the team’s leader, it's clear her head is 25% larger than everyone else’s, and her mouth operates like some kind of Muppet.
Where ODST really shines is Firefight, a new multiplayer survival mode where 4 players fight off wave after wave of Covenant troopers. Each wave gets progressively more difficult both with tougher Covenant and as new "skulls" become active. Skulls add new challenges to the waves (such as melee attacks being the only way to recover your stamina). Those who played Halo 3 should be familiar with skulls; but beware, in Firefight you have not choice as to which skulls are active. If you liked Gears of War 2's horde mode, or enjoyed Left 4 Dead, you'll love Firefight, but be warned there is no matchmaking service for Firefight at the moment so you'll have to play with your Xbox Live friend
Halo 3 ODST is, when you boil it down, just more Halo 3 with a slight twist. It's like adding a shot of hot sauce to your pizza. This isn't a bad thing, and with the addition of Firefight, ODST is definitely a package worth picking up. After you blow through the 6-8 hour single player you'll be sure to spend hours on Firefights. If you liked Halo 3, you're sure to enjoy ODST.