Wow, that's an incredibly satisfying game! I'm really impressed by how such a simple game can be so enjoyable to play. It's a top down shooter with a real challenge, which I feel most games of the genre lack.
My suggestion would be, to make the blue colour a bit brighter, as I had trouble seeing it against the dark background. And to not kill the player upon touching the sides, as I found I had so much to keep track of already, that I would hit the sides without realising and it was a frustrating way to lose a good run. Maybe have the player bounce off the sides instead?
Below is my highscore :)
P.S - GameMaker boys unite!