I stopped playing after completing wave 4 because I've went through enough of the enemies and gameplay to say that this was a really fun shooter game. I do feel that the shooting is slightly delayed longer for the player but other than that, the challenge was really rewarding for me.
Various enemies attack in different ways, player can regenerate health if they're not hit for a long period of time, and heck even the upgrade system is working and adds more flavour to the game.