Running the Linux x64 version. Fullscreen mode doesn't seem to work right, it just unfullscreens itself when the menu shows up.
It feels good to play, but I'm no good at explaining why things feel good to play. So instead, here are the things I'm not happy with.
I don't understand what the half-circle powerups do.
Mashing the button to fire at the maximum rate is annoying. There's no reason I should have to do that in a game in 2015. In fact, I'm not sure there's ever a reason to NOT fire (except at the very beginning when you can only have a few shots on screen at a time), so I wonder if a fire button is even necessary.
I think you need to be more deliberate about contrast - make sure the enemies and their bullets, at least, stand out against the background. Otherwise, I can't really see them unless I'm looking right at them. In particular, purple on brown doesn't work so well.
You have enemies coming from all sides of the screen, and they move very fast (sometimes faster than the player's ship). Short of memorizing the level so you can anticipate them, there doesn't seem to be a reliable way to avoid them. Maybe I'm just not good enough, but it doesn't feel right to me to be killed by a collision out of nowhere. Perhaps an arrow at the screen edge indicating when enemies will spawn would alleviate this?