Really enjoyable
Kept me entertained, had fun weapons, and I felt like I really enjoyed it. I can't find anything wrong with this game. In my opinion, that's a 10/10.
Also, here's the trick I used to get to level 35, where I quit:
Run at either the bottom or the top, near the right side of the screen. Shoot the zombies behind you primarily, then the zombies to the side of you. When the zombie clusters allow a vertical opening in the middle to the other side (top or bottom part of screen), run through it while killing zombies. Go to the right side of the screen. Repeat.