Addictive. I got about 30 minutes in and I was wishing the play would evolve a bit, but it still kept me hooked through the end of the demo. I love the upgrades screen. The art is all clean and precise. You also added all of the little polish flourishes like the way the upgrade info screens pop up with animations etc.
For the camera, building off the comment from Linglou3, if you didn't want to completely re-design it for ammo, maybe a button or upgrade to be able to toggle the rotation direction.