Ok, granted I only played the demo, but I thought it was just ok.
I love zombies. I have some weird obsession, it worries my wife. I prefer less action and more suspense. I was never scared of the zombies, they were just fodder. I never felt in danger or anything. I think this is more of an action-oriented game for people who have friends over. I can see how that would be cool. It's probably because I'm so picky about my zombie stuff, but I don't think I'll buy it. Weird though, because I don't think I've disliked anything Valve has ever made. Portal was brilliant.