Great game. Perfect length.
I love the fact each mission has it's own text. You could have easily made it repetitive, it's a small a touch, but it makes a difference and makes it a pleasure to continue you, you don't feel like it's a meaningless grind.
The story is simple, but that's perfect, it doesn't need to be anything more than it is.
My one complaint is that some bosses seem too easy. Easier than some of the ships I face in the level itself. It's probably because they're so big I always hit them, but they go down in 4-5 hits. (see: the third last boss). Apart from the final one (loved it) they're also quite similar, and so lack challenge. It's a minor thing, otherwise it's very hard to find a fault in this game. Great work!