Very smooth controls. Excellent variety in enemies, both in terms of appearance and behavior. Good pacing and escalation of difficulty.
Superb visuals, including the ships and the backgrounds.
A couple of very small nitpicks:
1. The music is catchy, but at times got very "noisy."
2. I couldn't tell what all the power ups did.
3. I didn't see a health bar for the boss--did I miss it?