Good, but lacking a lot
Overall it was a average game. It was fun, pleasant graphics, and a simple yet addictive gameplay. The only problem was that it had too much simplicity and lacked some addictivity (if that makes sense). Two upgrades (well only one UPGRADE to be technical and kind of a douche) just isn't enough to make a game high-quality front page material. Personally I don't think it deserves a score over 3.5, but that's just me.
The gameplay itself is ok, but people need relatively good video cards to handle the later levels smoothly, as well as notice a difference in the gun fire rates. After about 7 gun rate upgrades the game just gets easy and it can play itself. Unless someone isn't aiming at the zombies, it's impossible to lose at this, and that kinda sucks.
Still, I applaud your effort and hope your future games are a little more successful.