I had a really good time playing the demo! The level design gave a lot of freedom in exploring the world, there was enough ammos and resources scattered around to make the experience balanced, the secrets required a certain amount of attention but aren't as cheaply hidden as they can be in classic shooters, and the dark yet colorful graphics give a nice retro atmosphere - a sort of mix between Quake 2 and Doom 64. Enemies have some cool designs and the AI does a nice job keeping you on your toes, especially on the hardest difficulty. Finally, the weapons feel good and it's all so satisfying to blow monsters into gibs with the TNT or grenade launcher! My only slight bother was the heroine's sound effects: her grunts either sound too high pitched or too deep. I'm not fond of the head for the health icon either (it feels too much like a model out of Reboot), but that's a real minor detail.
An extremely good impression overall. I'll be following the game's progression. Good luck with the development! ;)