I got quite bored after the fourth or so level. The game just doesn't have enough substance to hold my interest. The upgrade system lacks much depth, and the level design is both bland and tedious.
On the plus side, the boss battles were pretty well designed. I just wish I didn't have to slouch my way through the rest of the level in order to fight them. I also didn't care much for the 'POWERUP' concept; I would have preferred that collecting the letters would automatically fill in any missing letters. Better yet, I would have liked to just fight the bosses one after the other without any interference, since each boss upgrades your weapon when you defeat it anyway.
Oh, and mashing the mouse button seems to cause a glitch which increases your rate of fire, which made the game laughably easy. Might want to get that fixed.
I'm a sucker for shooters, so I did enjoy the game. The design just needs some work.