Difficult but very, very fun!
This is a really excellent flash game, and I've been wasting more time on it than I really should be. Things to praise:
First of all, the controls, especially the manual, are way more intuitive than everyone has been giving them credit for. Just move away from the boss, line up your shot, and hold down fire to strafe. The Auto is a little clunkier, but after two games I was already getting to double digit stages again, so I'm not sure what the problem is. More than nine firing directions would be nice, but it's not really a necessity.
The graphics are fun in a retro-ish sort of way, and the music provides a nice ambiance, though I appreciate the option to turn it off.
The multiple "heart" types really up the "OH SHIT" factor after you get to them, and the free-form enemies are, of course excellent. One last piece of icing on the cake is that the achievements actually get you something instead of a shiny badge.
One thing I will agree with the majority of the commentators on is that this is a HARD GAME. I still haven't gotten past the thirteenth level. Nonetheless, for the more hardcore bullet-hell fan, this is definitely worth a look.
tl;dr: Overall fun and innovative game. Controls take a little while to learn, but make for a fun, if hard, shooter game.