Not bad, but easy
I did enjoy playing this game for a while, but I found that it was too easy. The difficulty levels didn't seem to increase the difficulty enough; while I did notice the differences between easy and hard, they weren't great enough to actually make the game any harder. Perhaps my biggest complaint is about survival mode; basically, the player goes through the boss fight over and over again, with no changes. I could do this all day if I wanted to, with one exception: the bug I ran into that ruined everything. I had finished three full run-throughs of the boss on survival mode, had a score of ~172k, and I just wanted to die so I could get the medals. When the boss loaded the first stage of the fourth run-through, I deliberately ran into him. This caused him to instantly die and progress directly to the third stage, but it glitched the game and made it impossible for me to move my ship or to close the upgrade window. I can hear the little drones shooting at me, but since I can't move my ship, I'm not getting hit by their lasers, and thus can never die. It's frustrating when I went through the fight three times in a row, only to get screwed over because of a glitch.
Anyway, the game wasn't bad other than the glitch I ran into.