Most of what I want to say has already been said (mouse aiming, customization, etc). Some of the enemies were simply too powerful to waste my time shooting (the ceiling-mounted "volcanoes" that dropped the yellow slime took 40 hits (!) each to kill). Other than that, I played it through to the end and I agree that this is a well-designed game.
Level 4: You're telling me a human can be shot in the face 12 times / chainsawed in the face and survive, yet they smack me with a club 3 times and I'm dead?
 "Pipe It 2D MOBILE"){kind=icon}
 "The Breach"){kind=icon}
 "The Final Death Wish"){kind=icon}