The game itself is pretty fun, but there are some pretty major issues that need to be addressed:
1. Every single level is exactly the same, the only exception being the length.
2. Money came in way too fast, while weapon upgrade points came in way too slow. The levels got to long for me to even bother playing the same exact level, at progressively longer lengths simply to increase the damage on a gun. Meanwhile, I unlocked every gun by level 7, while I continued to collect useless money.
3. During some levels, I would start taking damage when there were no enemies on the screen. A minor glitch which became incredibly frustrating, especially when these levels became 5-10 minutes long, and I was already half way through them.
The game seems to be well-designed, but no offense -- it doesn't deserve the 4 I accidentally gave it.