I really enjoyed this game, the graphics were quite simple but they fitted together pretty good, I can't say anything about sound/music because I muted it right away (Not because of the quality, I mute them in every flash game).
The concept worked pretty well, but it was in my opinion a bit too easy most of the time (except for the fourth and last boss, they were pretty tough). I only needed two lives to complete the whole game, having full shield/hull most of the time, so I would suggest that you increase the damage that the player takes.
The game was pretty long, in fact it was a bit too long here and there - referring to the highscoretable I'm the only one that completed it right now, so I guess most of the players will quit before the end.
All in all I really liked this game, I especially loved the slippy controls - I don't know why but I kept sliding left/right the whole time because I liked the feel of it.
Keep up the good work,