Really tight game! You have a great eye for game design, each boss and enemy type is engaging and conceptually fun and interesting! All around it's really solid and fun. I love it!
one small thing- the earliest enemies, single lines of about 8 enemies are a bit too easy to shoot down, since i only needed to hold down the fire button and aim in one direction. I understand early enemies should be easier, but at least these could require a little more thought/attention from the player.
Seriously though, it's a minor thing. It's a great game :)