This is a very fun game. There's no storyline or anything such, but one isn't needed in this game.
I like the upgrade factor in this game and the wide range of weapons you can choose from.
It's also fast paced, which I liked very much.
The only downsize was the switching between weapons, (but luckily everything I ever used was the evil smiley face and the poison weapon).
Can't really say anything to improve the game, maybe make an option for people that wanted to play longer.