Not too bad.
I do have a lot of little problems with this game. The reloading system isn't that good. Sometimes when shooting at crows you accidently reload. Plus the ammo system isn't great either. It's realistic, but takes fun out of the game. If you don't want to loose any ammo, you must use of the whole clips. You should use individual bullets. For example: you got 20 bullets left in the machine gun and you got a free second. Sure would be nice to be able to reload and get ready for the next horde. But in this game, you would loose those 20 bullets to reload. You should do something so that you get the extra 40 bullets you need and not loose 20 just to get a whole new clip. The controls could be better for selecting weapons and powerups too. I don't know, just got annoying with them during the game.
Other than that long list of complaints, the game can be really fun. You just need to make some improvements.