Great, addictive game. I like the idea of the game gets harder as you upgrade your shotgun, so you could incidentally not upgrade your shotgun and the game would be a lot easier. But then again, where's the fun in that? ;)
The music suits the game perfectly and had me humming along as I blew the brains out of a zombie, which is a first for me. The controls work well, the graphics are fantastic, I think if they were more realistic the game would lose some of it's.. zazzle.
A must play game, definitely worth the 5/5 I gave it.