It's pretty good. After a while of playing it I realized that you have to scramble to gather up the energy, otherwise the game will just keep getting harder until it's impossible to survive. That makes the game a good balance of avoiding the enemies and getting the energy cubes. I like the insults that are put in here and there, they made me laugh a bit. Furthermore, the voice acting is really good. It makes it seem like the game really is trying to defeat you. Very nice indeed.
However, there are a couple things that could be better. First, I suggest that you give a description on what each power up does in the How section. It may help if you're trying to come up with a strategy to survive. Second, I would suggest allowing the player to purchase the highlighted powerup by clicking the mouse as well as using e and spacebar... for some reason I think that would be more natrual as far as controls go. Also, I would suggest making your square brighter when you run out of health. Right now, when you are really low on health, your square disappears. That makes it virtually impossible to dodge enemies because you can't see where the boarders of the square is. Still, nice game overall.
Oh, and for those wondering, here are a list of the powerups and what they do:
Plus Sign - Restore some health.
Down Arrow - Destroy all enemies on screen.
Left/Right Arrows - Game changes conditions in your favor the next time the energy bar is full (or adds 1 to the number of times it will do so in the future.)
Plus Sign (with superscript 6) - Equal to 6 restore health powerups.
3 - Game changes conditions in your favor the next 3 times the energy bar is full (or adds 3 to the number of times it will do so in the future.)
Also, note that the left/right arrows and/or the 3 powerup will always remove the health siphon first, if it's activated...