After playing all the way through the two Demo levels and also a match with some Steam friends, I'm still a bit undecided on this game.
I think it has the potential to be something great, but server situation is gonna have to get a lot better before that can happen. None of the servers I got onto had decent ping, and that was a major pain in the ass to shoot zombies only to have them die 2 seconds later.
On the other hand, the combat mechanics felt pretty tight when I was in a lag-free single player game.
During a multiplayer game with a few buddies, I got a good feel for how the social aspect of the game plays out in groups...there were a number of "holy shit" moments where we were scrambling to keep each other alive.
But until you can have both tight, lag-free control and the social aspect at the same time, the game is pretty gimped.
I assume that this will be addressed soon, once more reliable servers go up and Valve starts giving players more control over which servers they join.
I also think that killing the run-of-the-mill zombies could get old pretty quick. The boss zombies are pretty cool, but more is needed. I hope they end up adding content on a regular basis, the same way they have done for TF2.