Epic game, I'll say.
I was doubtful at first, thinking this would be another of those "you just go around bashing the hell out of zombies." Glad I'm wrong.
It was an easy to understand game, the story was simple, the music was awesome and it was surprisingly enjoyable beating the ever loving crap out of the zombies.
Sadly, it was only seven levels, which was kinda dull. I'm aware that Survival Mode is 99 levels but I highly doubt I'll be able to pass that any time soon...
Also, you might want to make the player able to buy some upgrades with the money you get. I liked how I'd pull of a very high combo and have a ton of cash, though... Admitting, I payed more attention to the money than the zombies (didn't die, though!)
Excellent game.