People have been unlocking this early on Steam via VPN's and suchlike, and many of them are complaining about poor performance, horrendous load-times (60s+ just to return to the menu!) and constant crashing.
You can read about about this at
Neogaf or check the
Steam forums to see what people are saying.
Total Biscuit says in a series of consecutive tweets
"I can confirm that the people claiming performance is shot to hell on forums are correct. The game runs poorly on my system. The game also suffers serious mouse acceleration issues while in aim mode. SLI support is not available right now either so your second card wont help. I'm going to test it on my second machine today to see if those results continue to be the case. But the game appears poorly optimised. As far as I'm concerned the game is not advanced enough to justify that poor performance. Texture quality is a mess. Even with very few zombies on the screen it's still tanking frames on my PC. I'll try and do a port report today after I make sure it's not just this PC its having problems with."
As always, don't pre-order games and wait for reviews/impressions before buying. At the very least, wait for a patch. This game is currently broken.
*edit
reddit