The game is quite simple but also pretty addicting. I spent quite a while running around and dodging zombies. It's actually harder than I expected - you run out of ammo pretty fast and oftentimes the zombies get to the ammo packs before you can. I also like how the knife makes you move faster than the guns do, so it's not totally useless.
One thing I'd add is a mouse cursor so that it's easier to see where you aim. I'd also add in a hold-to-fire function so that you don't have to repeatedly click the mouse to shoot.
Great game, I had a lot of fun with it.