Not a bad game at all. Crisp graphics, a good sound and music set-up, and clever idea. However, even on the hard difficulty, it still comes off a bit easy due to how overpowered you can become with the right item pick-ups. Also, the prices even on Easy get ridiculously expensive (jumping from 200 to 700 for one upgrade? Seriously?), though I do understand we have to work to get those upgrades, but sometimes it feels like you raise the bar so high early on, it's ridiculous to get there.
Also, a better AI gun would be better than the anti-air sphere. When it starts launching two electric spheres at the same time, it really starts to slow the game down a bit, and the rocket upgrade also tends to make the game lag with the constant explosion stream. Maybe smaller explosions for the smaller enemies would help?
This is a great game overall, with a few marks against it. Hence my 4.5 out of 5 review.