Nicely done! Took me a minute to get used to the slower moving missiles, but the controls were tight and easy to use. The difficulty ramps up a bit slowly, but it gives the player plenty of time to get used to the game before getting down to business.
The only nit pick I have is that you might want to turn the volume on the sound effect you have for the missiles firing. You end up hearing it a lot with how often you fire missiles and it started to get on my nerves after awhile.