I tried the demo, and overall I like it -- especially the atmosphere -- but I have a few concerns:
§ The controls seem slightly unresponsive, like there's a slight latency on input. Not enough to be a deal-breaker, but still noticeable.
§ The game seems to choke very easily, with just a few objects (enemies/bullets) on screen. Even without any enemies and just a few bullets there will be slowdown. You might want to limit the number of bullets allowed at once further to avoid this...
The above two might be related to GBstudio's performance I guess, and if so difficult to do much about...
§ When firing rapidly, maxing out bullets on screen, the firing sound will play even if no bullet is fired.
§ The way the player is turned around when damaged is very disorienting. Though I guess this is intentional, and you might get used to it.
There was even worse latency -- significant latency -- when playing the browser version, but it might be related to me being on Linux+Firefox. Still, it might be something to consider as it doesn't put the game in its best light.