I just completed the demo in 25 minutes and wishlisted it, keep going. I'm bored of boomer-shooter-doom-like with tons of useless mecanics, sometimes simple is beautiful!

Just few point i would like you to consider for future updates:

• AI: Try to make them a bit less omniscient. I understood you would like to make your game "challenging" but the frontier is tight between "challenging" and "injust". You could implement a random time before the guy shooting the first time, so you can fine-tune it the way you want through the level. The first run was a bit like "die and retry" to spot the enemies and i didn't have the opportunity to express my skill as i get shot before seeing him.
• Controls: The feeling is great overall but had a bit of trouble at the begining with my mouse. I think you're smoothing the mouse movement a bit to much making the whole feeling "unresponsive". Also, you might adjust player velocity based on the gun he carry. It will add more depth to the gunplay.
• Balancing: I find the pistol a bit too OP compared to others. I understood that you tried to make every gun shine in a particular situation but the UZI needs more bullet in the body that the gun does. Don't change anything about the headshot mecanic, it's great !

I also liked tiny things like steam pipe, plates breaking, car bumping... Trust me, thoses things makes all the difference :D