Absolutely fantastic. I have played arcade shmups that were less airtight than this!
A couple critiques, though: The unlockable drone weapon (the "box of doom" one), is pretty underpowered. Considering how hard it is to get the buggers inside that little box, it should do considerably more damage than it does. I bought it, and after playing one stage with it, went back to the missiles.
Thoroughly enjoyable, the art was gorgeous. My only beef is that the end cinematic wasn't a bit more epic. 4/5, you've got a great aesthetic going here. Sequel please?