Well, this is definitely a great shooter. I love games that just let you annihilate stuff, and this is no exception. The upgrades are fun and well balanced.
Unfortunately, there are two problems with the game - for one, there's only one type of enemy, so the difficulty essentially comes from there being more and faster enemies. So I suppose that a few new enemies would be nice.
The bigger problem is the whole doubling thing for upgrades. Doubling your points takes a long time, especially at higher scores and half the fun is from getting to improve your ship. So by making upgrades come rarely, it makes it less fun. Perhaps you could add 10 to the required each time, instead of doubling - I dunno.
A final note that comes as a result of the rare upgrades is the recoil, since it feels like you get relatively few upgrades, so you want to spend them on the seemingly more useful abilities. As a result, I only spend points in the firepower and rate of fire, since it seems like the shield, speed, and recoil increasers won't help me kill stuff. So it eventually gets to the point where you can't move because of recoil... but maybe that's just me. Maybe if I spent points in it, it wouldn't be as big of a problem, but then I can't kill more stuff! I dunno.
Either way, more upgrade points would encourage spending them in different areas. So apart from that, it's a fun game, and a great time waster.