Eh.
It was fun at first, but by level 4 it started to feel like a chore, and I was playing on the highest difficulty setting. You boast 10+ different weapons, but I don't really think that's enough here. 20+ might be more like it, but that's just speculation because I only managed to unlock one of them.
Game might have more lasting interest if the weapons didn't have to be unlocked (or if they were unlocked much quicker), if you could perhaps hold two or three weapons at once (allowing for more strategy), and if the levels were more varied (like maybe including interactive stuff, hazards, different terrain types...).
Also, nice coding. My computer is really slow and I didn't even get any lag at all even when the zombies were mobbing me.