Potential, but ultimately nothing but frustrating
I have just spent an hour trying to kill these zombies... which theoretically is a simple thing to do... if the targeting system didn't glitch every second or third headshot. I mean... I sometimes wasted both my secondary and primary weapon ammo, just trying to take down the basic zombie that was standing still on my porch... it wasn't moving and the targeting reticle was right on top of the creature. I wasn't even trying to headshot the sucker (which is a painstaking and frustrating thing in and of itself). I dealt with that everytime I played... and I feel that it really detracts from the gameplay and the enjoyment. Not only that, but your save feature doesn't work. I saved after every cleared wave, only to lose and not be able to reload my game. It wasn't very fun having to play all the way up to wave seven each time... only to die and find out: hey! you were one zombie away... but sorry, start over.
So the frustration and the glitching is the summation of your score. I had no fun after the first five minutes.