That was a really solid package all around. Good art, good controls, good music, great gameplay.
I love the mechanic you went with for the power ups, where you have to accumulate a few in order to buy the higher end boosts. This adds an element of risk vs reward that I thought was nice.
There are a few things I think could have improved the experience:
1. The fact that dying resets your power ups to 0 means that dying late in the level is very, very punishing.
2. Related to this, I wish you could occasionally get power ups during the boss. Dying to the boss means you would have to plink at it with the basic shot for a very long time. (This happened to me, and dying after shooting at the boss for a good 30 seconds I'm pretty sure was a very frustrating experience.)
3. I was surprised to find you can only upgrade up power ups up to twice, and ended up losing a late game chance to upgrade my speed. I think displaying the powers up with, for example ( ) ( ) -> ( ) (O) -> (O) (O), would go a long way into making it clear how much you can upgrade.
Also, while playing on Firefox, at times (especially when there were a lot of enemies and shots on screen), everything except my ship and the environment would become invisible. I did not encounter this glitch after switching to Chrome.
The title could have used a little more thought, I think. ;P
Still, over all, great game! I love how you handled power-ups here, aside from losing everything on death.