Really cool game, I think the shooting mechanics are unique and the it makes the combat with the aliens a lot more tense, having to wait for the opening for a quick shot! The atmosphere is also great, hearing the noise the aliens make before encountering one really adds to the suspense.
I think the levels could be a little smaller though, the corridors and rooms are a little too big for the visibility and it makes dying a little too punishing personnaly.
Good job!