Good, but too casual.
This is a very cool and fun game, but the problem is it isn't fully fledged. Gimme some actual levels and boss fights, whatever story you come up with is better than none. I think this would improve the high score system - the score you would get from completing every early level flawless is about the same as stringing together a few wins in the late levels - which is of approximate equal difficulty as in the later levels you have awesome weapons. High scores for each zone would make the scores more worthwhile.
I loved the selection of weapons, they had good balance and the timing of receiving them was well done. I don't feel the same about the zombies though. 3 types and no bosses made it a little repetitive, but the variety of weapons and horde sizes did compensate. i think, however, the big zombies should have appeared before the supercrawl zombies - the aiming in both directions with the limited range made a supercrawl zombie in front of you more deadly than a horde behind you.
The only other issue I had was that when continuing, you couldn't swap weapons. Other than that, this game is freaking awesome! Spent an hour happily demolishing zombie hordes and feel better for it. Cheers :)