simply put, this is an example of a good mechanic, put together with a genre that excels in, done right.
if half the games being made today, had mechanics in it done as good as this... well, it would be awesome.
the only thing that -did- get a little on my nerves was the graphics level, but considering everything looked ok, did it's job very well, and was varied enough to tell they were different, I couldn't really complain that much.
a special mention to your music choice, btw, both music pieces fit there places extremely well, even leading to some times where I could've swore the shooting was in sync with it (though was obviously not... was cool idea while it lasted though)
the only other thing I can think of... is you may want to watch where you have the bullets coming from... while it wasn't terribly bad this time, there was still a couple points where despite the boss being fully on screen, it seemed bullets were flying at me from magically nowhere... next time, simply have the correct amount of bullets fly off screen sideways or up, a couple seconds before the offscreen bullets appear. it won't matter gameplay wise, but it's the little touches like that that can get noticed while not take long or hard to do :) (the bullets dissapearing, however, was perfectly fine, as you had the white flash showing they were gone now)
ps: boss counter to show which boss was which, please? was happy I beat it, but also glad there wasn't another one because I had lost count dodging bullets... lol.