I played some splitscreen multiplayer with my brother yesterday. The racing mode is apparently not available if you're not logged into Xbox Live, but the cooperative option was available, so we gave it a try. Here's what we found:
On the Xbox at least the game's graphical quality is significantly toned down for multiplayer. The game drops from a fluid 60fps to 30fps, the texture quality goes down, and the level geometry is notably simplified. But despite these limiting factors, we found the game very enjoyable - level 1, the prison, was somewhat dull but levels 2 and 3 were very good, both in terms of general aesthetics and gameplay. I don't expect the multiplayer to last long, though - we managed to beat 3 of the 9 levels in just under an hour and they are all roughly equal in length. Therefore, while I anticipate future DLC, cooperative playing time only clocks in at about 3-4 hours. And there isn't much replayability, either: aside from achievements, the only configurable game setting is the difficulty, and there are only two choices: normal and nightmare.
In terms of the singleplayer mode: I have another tip for you guys. I don't want to spoil anything, but...
When you get to the Dead City, make sure you bring plenty of ammo. I ended up using the shotgun a lot, but I didn't bring nearly enough ammo for it and I ended up finding myself in lots of bad situations through that point of the game. Your pistol is useless there.
EDIT: I can't believe I used four colons in this post... I'm getting sloppy.
