Wow, that got really fun after level 1!! The slipping around and the angled bullets were a real challenge. Nicely done!

This was a very cool idea, but it was kinda short.

I think that two things (besides making the game longer) would improve the game a lot:

1- make a small pause between stages
2- clean the game state between stages. I jumped from stage 3 to stage 5 in a second because my bullets were in the right places.

I liked the sound, and I really liked the simple graphics.

Interesting, though the change in controls after the first level was... frustrating.

Additional:

Since I can't vote (not a participant), I'll give it some more in-depth feedback here.

Sound - 8 - Not complex, but not annoying, just the right feel for something that's aiming to be simple
Graphics - 7 - Though incredibly simple, things are consistent and it's easy to work out what's going on. The lack of complex graphics doesn't detract from the game.
Gameplay - 7 - If the controls stayed consistent, I'd rate this much higher.
Concept - 7 - Reasonable, though I'd like, perhaps to see some form of transition/change between the enemy to indicate their upgrades.

Overall: Great. Stripping things down to a simple look/feel means we see the gameplay itself, and it stands up well. Controls can be frustrating - especially when I transitioned between a level (with no indication of doing so) and was promptly slammed into a wall as the controls 'stuck'. The low responsiveness of the controls in later levels also hindered it. I feel that instead, if the game sped up instead of slowing me down - I'd get a much greater sense of Zone.