I like the game. Great idea. I love those zombie flash games. A few comments though:
It would be nice if the player had some little animation like the monsters. It would also be nice if the monsters had dying animations, they just disappear.
Shooting the Wacraft grunt hmm... Frankenstein :-D in the legs doesn't work. Bullets go right through. Same for the skeletton. The collision rectangle for the Cerberus could also be enlarged (Try shooting just the top of its back).
Is there a "heat effect" on the Uzi, because after shooting for some time, the rate of fire slows down dramatically?
Would be nice to have an indicator when you have enough money for an upgrade.
I had trouble clicking on some menu buttons too. They definitely don't work if you don't click right in the middle.
I didn't find there was a tilt option until I read about it here and checked the options (minor problem, but some of my customers seem to be real idiots about stuff like that). Tilt feels inverted for me so I would like the option to reverse it.
The sound loop is a bit short, got on my nerves quickly (but I turned it off).
Overall very nice game, with lots of replay value. (I maybe biased, I used to play "the last stand" all the time at work :-D ).
I hope you take all this as constructive criticism. I left a good review on the Swiss store.
PS: Oh, and a zombie shooting game without a shotgun is lame :-D