eh. Minor problems start to add up
The upgrade system could be much better,
the number of zombies starts to get tedious,
the game gets boring fast...... 5 minutes or less kinda fast.
As-is, it is nothing special and even a bit annoying. I'm going to play something else--but I think this game has promise. The weapons and upgrades need some rebalancing. The sheer number of zombies per waves needs to be reconsidered.
Some things to consider adding-- a very short tutorial level, the ability to eat the dead and regain health, and in general add a few things to make the game more original.