I love the concept, though it feels a little clumsy in execution - I'd have liked a default "gun," or perhaps more shots per "pickup" (e.g. each pickup gives more "ammo" but one unit of ammo gives you a single shot instead of multiple waves of automatic fire). As it is, I feel like most of my shots are wasted because I can't really control them.
Wild stuff with that arcade ROM bootscreen intro, by the way - I just wish that level of polish carried over to the gameplay!