Damn, now this was addictive! I made a shmup too, so glad to finally see another one : D
First game this jam that I couldn't put down until I had it beaten, and even then I wished it had gone on longer ^^
At first I thought the physics were a bit floaty, but I got used to it by the end. I find it particularly fun how the order of upgrades you take kind of determines the difficulty. My first idea was "more damage, sounds good", but turns out that's way less useful in the beginning than the grenade launcher.
Overall really solid entry,I think you could turn this easily into a pretty good full game!