It's Zombielicious!
Decent graphics, good sound, and great gameplay. The game engine is based on Unreal 2, and the game controls are your standard issue WASD for movement, R for reload, C for crouch etc. If you've played any shooters at all you can pretty much jump right in.
Wall and object textures are only average, but the character models, especially the zombies are very detailed. Some, like the half-eaten and decomposed zombies are quite hideous. Sound is very good; the weapons sound like they should, and the noises the zombies make will make your hair stand on end.
The music is outstanding, and is rather creepy.
In fact, it is so creepy I had to finally turn it off after the umpteenth time I jumped out of my chair when a zombie came at me from out of a hole in the ceiling, or popping out of ventilation ducts.
The demo I thought was too easy, but the full version of the game has several difficulty levels. Even on just "normal", the zombies will give you a run for your money.
The full game's first level is very hard, as you start with no weapons and frantically have to search for something, anything that will faze the zombies.
It had me one step away from full panic, the game is that good.
Multiplayer is a blast, especially the mode called "Invasion". In it, you and up to three friends must cooperate to survive wave after wave of the undead. What a hoot. The game also has the standard deathmatch and CTF modes, with the exception that in addition to worrying about human opponents, the levels are infested with zombies. While you're setting up that killer sniper shot on the guy trying to capture your "flag" (bag of supplies), suddenly you're being attacked from behind by a zombie. What a hoot!
The game does have a few small problems; occasional clipping, and my system locks up when exiting multiplayer (although this could be driver related). Hit detection is iffy; head shots on zombies can be difficult to make at times. Also, the zombie AI can be exploited; jump on a desk or chair, or crouch in a corner, and the zombies can't see you! This does not always work however. I guess some of the zombies' aren't "firing on all cylinders". But they are after all well, zombies, and not the sharpest tools in the shed. But all in all, this is a great game, and well worth the $20. I'd gladly have paid $40 for a game of this quality.
So all you zombie lovers out there, rise up out of your shallow graves, and shamble or shuffle off to the store and get this game!