During the last week, I got Halo 3: ODST. It's the first new Xbox game I've gotten in a good while. I haven't played Halo 3 much since the release of COD4, so before I got it I played through Halo 3 so I'd have something to compare it to. The actual campaign is kind of short, but it's well polished.
The first thing I noticed about was the price. At €45 it's a welcome break from the €60 new releases I'm used to. And definitely cheaper than MW2 which is going to be €70 at release here.
The graphics are about equal to those of Halo 3. So while they are far from bad, they're still outclassed by many other games. Given that Bungie has up until now mainly had to animate armour and aliens, some of the details such as hair and skin have a PS2 feel to them.
The story is where ODST shines. It makes a break from the "infiltrate big forerunner artifact, blow it up, escape" of previous Halo titles. You wake up 6 hours after the drop and have to track down your squad, with flashbacks to the events earlier in the day on finding each piece of evidence. I can't say too much more without spoiling the plot.
During the gameplay, there are a lot of "defend" sections. These are quite fun, and are long enough to be challenging without being boring. The game's special mode is a defense game where you have to last as long as you can, much like COD5's Nazi Zombies mode. It's more serious than Zombies, lacking the powerups (at least on your side, the skulls randomly activate to increase the Covenant's strength), and novelty aspects (ray gun, wunderwaffe) of Zombies, but replacing it with much larger maps and more variations in enemies. Despite much of the hype about the ODSTs being weaker than Master Chief, and a more tactical game, the only part you'll notice this is when you are fighting against the hunters.
Overall, I'd rate it about 7/10. I actually had more fun out of this than Halo 3 itself, which didn't impress me too much.