Not bad
It's a semi-decent time sink, but there is plenty of room for improvement. First, let's talk Zombies. I don't feel there is enough of them. By level 3 or 4, all the zombies were unlocked, which made the remaining time I played kind of bland. A giant boss zombie every few levels would have helped, and an actual end of the game as well.
Now, let's talk weapons. You have the basics down, handgun, shotgun, machine gun and some other uber weapon. Fair enough, but why does it cost me 2x the kills to buy the dual version of the weapon than it did to buy the-handed version? Yeah, I know that's kind of standard in games, but it really dosn't make sence.
This is more of a preferance thing than something I'll deduct score for, but why is there no Flame-thrower? Yeah, a chainsaw powerup is cute, but for my apocolypse, I'm ditching that tiny thing for a freaking fire-spewing monster. Zombies tend to burn very well, so if you made it as a powerup and had it set so if one flaming zombie touched another, the fire would spread, it would make a great weapon.