A lot of fun
I'm normally not a massive fan of 'zombie shoot-em-ups', however I did spend a good amount of my time on this!
Of course the levels were repetitive, however the stats at the end of the level (% headshots for example) and wondering if you were going to get a new weapon or not made it a lot more fun to play, although when I earned the flamethrower, I seemed to spent most of the time in the levels holding the mouse button down and running about and succeeding, which eventually got pretty repetitive.
Overall though, great job and best of luck with any future Newgrounds submissions