I've put off writing about this, because I wanted to have some non-zombie related news to post first, purely for my own sake of mind.
In essence, Dead Rising is superb. It's really everything a next gen game should be. It does something the previous generation couldn't - namely enormous hordes of the shambling undead - and straps it to a finely-honed game mechanic. It is not, by any stretch of the imagination, a complex game, but it does what it does really well.
I've seen it compared to Resident Evil, which seems a bit lazy to me. It is made by Capcom, and it does feature Zombies, but it couldn't really be more different. The best comparison I have for it is Spider-Man 2. It's a game full of simple tasks, and getting from one point to another is really the only challenge in the game. The point is, if you make getting from one point to another interesting and entertaining enough, you have a great game.
There are faults - ally AI is poor to the point of maddening, and the time pressure means you've very little time to experiment with the various weapons on the first run through (unless you don't care about saving anyone, in which case I reccommend stocking up on shower heads and just having yourself a ball).
At the end of the day though, any faults can be forgiven with the sort of quality on display here. Well worth getting a 360 for.