fantastic little game. I've played others that are very much like it, though I enjoyed this one just a little more.
I would've liked more options for weaponry and upgrades, but it works.
What I have against it - and this is a small point - is the genre; Zombies.
The Zombie genre - in all of it's iterations - has been (pardon the pun) done to death.
The premise is well-worn and it is hard to make an interesting Zombie game (or book, movie, television show, et al) So having chosen "Zombies" as your antagonist is a bit... disappointing.
I would have much rather have seen something new OR, if you are going to use zombies, come at the subject from a new and different perspective.
That being said, this game works and I enjoyed it quite a bit. Not "hard" but enough of a challenge to keep me interested.