Wow, I like this a lot.
I played this quite a lot, not the best at it, but got to the three star diffilculty, and my name up on the highscore board. Let me not be the first to say that I liked the graphics, cool and more original gameplay, and the weapons and powerups, etc. The endless gameplay was one of the most important features, because it keeps you playing until you can't do any better, not just until the game ends. The new powerup feature was interesting, but still worked well in the game.
Ok, now for the other stuff. You gave us a choice of four weapons but how come you can't use the last one (the shock or something)? Or do you have to get to a higher diffilculty than three stars? When you shoot enough enemies you get to power up right? Well, it gets annoying to click that red button on the bottom of the screen when you're trying to fight your way out of a giant mass of enemies. I would have liked a hot key for the powerup screen like pressing p or something like that, but you most likely didn't use the keyboard because you wanted the game just to use the mouse. But if you didn't want to use the keyboard in gameplay, why not just make the powerup screen come up automaticly?
Now, towards the actual powerups, most of them I didn't use a lot, and some weren't much use like damages enemies after destroyed. But on the flip side, I had to depend on faster reload every time during the later levels because of the speed the enemies came. I would have liked the waves or levels to be longer because since powerups that don't stay towards the next level, getting them during easy levels seems wasteful unless you had a while to really use them. I also may have liked powerups that effect other things like enemies move slower, or weapons have half range for a certain time. These may not be activated by the player, but they could mix up gameplay during the game and make it more interesting.
A quick but minor glitch I found was that if you are reloading and press the button to powerup, the bar that shows how much time left for reloading is gone. It doesn't sound too bad, but it gets annoying when I don't know why I can't shoot for a second. I also didn't understand why the enemies were attacking, can you add a quick plot of some sort to explain, or do you not want to because you want this to feel like a fast pased arcade game (which it does).
I really love this game no matter what though, and I would buy it in a second if it were for mac computers too.