Very nice!
I scored over 7000 and got 3rd place on the high scores!
Anyway, the game is brilliant - great powerups, good concept and well designed.
If I had to pick out one thing as the weakest part of the game, it would be the shield/lives, because it is so easy to avoid dying for a very long time, and extra life powerups appear too frequently.
Also, a boss after about 10 or 20 waves would have been nice.