Overall, pretty good. nice graphics, smooth controls, each enemy type felt reasonably unique, and of course the concept itself was very easy to understand. My suggestions:
First off, once the player gets all 3 specials, the game becomes very, very easy. There is almost literally nothing the enemies can do. All the player has to do is keep one special going all the time and (s)he's basically unstoppable. Maybe make the specials more expensive or increase the charge time on them?
Second, a repair "upgrade" is pretty much essential, otherwise there's nothing to spend money on late game and it prevents early game mistakes from becoming game-breakers.
Third: I'd recommend throwing in a few more durable enemies, something that appears less often than the other enemies but can survive at least a few seconds of exposure to a special, just to keep things more challenging. Maybe like a mini-boss? Just a thought. I'm addicted to boss battles.