Quite a good game
Ok, I'm kinda short on time right now, so this is probably gonna be blunt and wordy:
First of all though, I'd like to bring some insight to the "bug" that people keep referring to - flash, in general, has a tendency to keep the last button, whether it be keyboard or mouse, pressed once you do a click inside a flash window and release the mouse button outside in games like this which auto-pause when you get out of the window. So, if you want to get stuck, click the mouse button, hold W down, bring mouse outside of the game window and release LMB. Then return and click resume game - there you are, nice and stuck on the left edge now. Just tap W again to get unstuck.
Now to the review:
Pros: Perks system, graphics, overall gameplay
Cons: Upgrade system, Prison keeper
The good stuff is obvious: lotsa perks, different effects, gotta think and build a strategy. The graphics and animations obviously speak of a lot of time spent at the drawing table, though my eyes too would be a lot happier with more distinct dead enemy frames, and increased contrast between enemies and the ground, in my case, this applies especially to the lava levels. Gameplay was fluid and rather enjoyable,
The bad stuff is not necessarily that bad - it's just that the idea of dropping certain weapons only through monsters that are scripted to appear at the end of only some levels means that the player might never get those weapons, and without the ability to re-visit those levels freely, this is not a good design decision - this makes the game less enjoyable on a single-run sort of play, I mention this, because if you miss a weapon, you're out of luck after that for a good while.
The prison keeper is another issue that would be easy to solve, the strategy suggested by the author would work, but the issue is not necessarily the strategy, the issue is that the random number generator simply does not generate as many little bots to get powerups from to make it possible to survive - every time I played this level, I managed to kill only 3 little bots before I got killed, even doing nothing but trying to hunt those, the keeper found me long before I could find a fourth one to kill.