Nice graphics and music, annoying gameplay
The difficulty is really cheap...
Your hit box is waaay too big.
The bullets are so small you can barely see them sometimes, and later on they move so fast you can't dodge them, unless you're not in their way to begin with.
Enemies pop out from the bottom of the screen without warning, and sometimes at lightning speeds.
Often my ship simply isn't fast enough to dodge some wide attacks.
After getting hit, your invulnerability should last longer. Foes shouldn't be able to hit combos on you.
The upgrade system was pretty cool, but maybe you should have left speed out of it.
I know you probably expect players to memorize the levels in order to dodge stuff, but flash gamers aren't serious enough for that.
More people would enjoy a game that looks harder than it is (tiny hit box, tons of bullets), than a game that's harder than it looks (this).
Anyway, good job on the graphics again, especially liked the backgrounds.