Agree with the other comments that the aesthetics really stand out. At first, I was a little concerned with the difficulty given that the bullets only came from one direction, but I like the evolution. In particular, I think it's really interesting how, instead of just adding the obvious bigger and faster bullets, there are big, slow bullets. They do a good job zoning off areas of the screen and giving you space to dodge the smaller, faster ones. Really neat!
I also agree with Delovoid that the stages could go by faster, with a faster ramp up in difficulty, but I had fun anyway.
Great job!