I really enjoyed this game! I'm a big shmup nut, and it was well-designed and the graphics and music are rad! I do have a few minor criticisms though:
1. The elongated bullets that many enemies (particularly the first boss) shoot look incredibly cool, but I think they might be a bad idea from a design perspective. They're not large, but they can easily form nearly-impenetrable walls and are much more dangerous than even a whole bunch of normal circular bullets. You should use them more sparingly, I think.
2. The enemy types and flight patterns are a bit dull. GameMaker's path feature is super easy to use, so there's no reason the enemies shouldn't be moving in more complex formations! And you dont' need to hold off until the end for that cool stuff, either; even from the first level I was a bit put off by the fact that enemies always just flew in a straight-ish line.
3. There's not enough incentive to use the sideways gun. I would suggest copying the excellent Radiant Silvergun, in which the sideways bullet does much more damage than the other guns to counteract its difficulty of use!
Despite these nitpicks, I think you've done a great job with this game, and I'll probably play the updated non-submission version in just a bit! Congratulations :)