There's a great interview with one of the top guys at id on teamxbox.com (sorry I don't have the link for you), and it answers a lot of questions that have been getting asked a lot. Here's some info. that the interview included:
-Type of game: Straight first-person shooter. This is NOT an RPG. You do not upgrade your character or weapons. You do, however, have different ammo types, armor, and dune-buggies that you can acquire and use throughout the game, which somewhat influence the gameplay experience.
-Length of the game: 15-20 hours, depending on how much you explore and how many side quests you complete.
-Co-op: Online and split-screen. There are missions especially designed for co-op; you won't be able to play the entire campaign cooperatively. You can also play these missions alone.
-Size of maps: There's the main Wasteland, which is a pretty large map. The "missions", most of which are 30-45 minutes, all sort of branch off the Wasteland, similar to Borderlands or Fallout/Oblivion. But the Wasteland won't be anywhere near as big as those games.
-Installing the game: IT IS NOT A NECESSITY FOR PLAYING THE GAME. Nor do you have to install all 3 discs at once. Just install whichever disc you're currently using. The difference will be a little less texture pop-in for those who installed the game. Installing is always a good idea because it reduces the wear and tear on your xbox.
Those are the key points I took away from the interview, I recommend checking it out for yourself, or you can also ask me any questions and I'll try to answer them from memory according to the interview.