I got it at the midnight release So far my favorite part of it is the new Firefight mode, where basically your on one big map, and you fight off waves off different enemies, it's almost like Nazi Zombies for halo :D
Meh, there are differences. FireFight is probably the most fun mode in the game in my opinion. But yeah, it pretty much is the same as Halo 3. I haven't been a fan of the story mode, I usually only play the online disc <3
I heard that Halo ODST has a really good graphics and concept, but the creators could of done so much more with it.
A friend of mine played it over a night and said him and a mate finished it within 2 - 3 hours.