Although i have to admit the game is fun, I personally haven't liked the concept of the game of "give life to gain more power" (actually i dont like the concept of "give X to receive Y", but that if it's well executed it can still work out). And talking about power: when you upgrade your power, the actual damage doesn't really seem to go up, it just increases the ball size that, yes it increase the range of the attack, but it also increases the time you have to wait for the ball to charge. So you're really trading life for less precision, which imo isn't a good trade, because it's the equivalent of saying "Hey, you are having a bad time with the game?? If you trade me 1 life point I'll make the game easier for you, giving your attack more range", and this wouldn't have really been a problem if there was a way to regain health (and you was trading on Max Health instead, otherwise your concept idea will break), but nope if you get hit is one less powerup for you, better luck (well, skill really) on next run. And on the late waves (5+, idk if there are more then, i didn't played for that long) the power up they almost feel like a requirement for how many enemies there are on the screen, but at that point you probably won't have enough life point to buy one so it becomes a game of "Hit & Dodge, or Die trying"
Although i can seems annoyed and like i didn't liked the game, i'm not: The game was great for the 5 minutes that i played, and the music that get more frenetic/calmer when there are enemies or not on the screen was a nice touch. It's just that i wanted to share with the developer how i feel about the idea, since in the comment i didn't see mention of this whatsoever (At least, in the front pace, this game has SOO MANY comments i couldn't possibly have looked at them all)...