Just finished a run and wanted to give you some feedback. I enjoyed the game and I had some genuine fun with it. I did however, run into some things that I wanted to mention.
I love a good screen shake, but I felt that it was a bit much as you are constantly shooting.
I think the game can be a bit too easy. I ended the run simply because I couldn't be killed. I wasn't losing any health, and my bullets were slaughtering everything in my path. Even when I did get hit, I would regain the health almost immediately.
After a good while in I found a pretty game breaking strategy. If you hover over near the edge of the block on the right end of the room, the zombies never seem to spawn behind you. With a couple of points into the penetration and damage perks, I could mow the zombies down almost standing still, as they were funneling straight toward me. Occasionally one would spawn behind me, but it was easy enough to deal with and turn back around.
The art is great and the shooting feels very nice! I would love to see this with more levels and definitely more guns!