Played it today, here's my small light review.
It's brilliant, the movement of the zombies is horrifying and when they run at you it's truly terrifying to a certain degree in some cases. There are a good variety of zombies too, and i am pretty sure you're always on foot with your AI or (preferably) mates on LAN/Internetz. You really feel like part of a team as well, it's quite brilliant, and jumpy too. Weapons feel good, though i've only tried a few, and there is (as always with good Steam games) a list of challenges you can achieve along the way.
As for online coop or versus, i don't know yet but apparently THAT is supposed to be the highlight of the game, and if it is, then we have an amazingly zombietastic game on our hands. I highly recommend it.
Now, i could only see fallout 3 in the shop today so i bought that.. but GET LEFT FOR DEAD ALSO!