So, I just finished the campaign of ODST. We played 4 player co-op, on normal so it wouldn't take all night. That having been said, it was really short. Three and half hours which included a lot of cut scenes and walking. The flashback thing is ok but doesn't make sense at first.
The wireframe/nightvision thing is really neat, although you can't use it outside because it is (obviously) too bright.
The SMG has a silencer and is sweet.
The pistol has a silencer, and is more like Halo 1 in which it can scope and is criminally good for head shots.
What else...
We didn't mess around with firefight/multiplayer yet.
Overall the gameplay is really awesome. The graphics are a tad bit better than Halo 3. Like I said the campaign would be longer on legendary, but I still feel it was too short.