Pretty good, if you like this sort of thing, few things I noticed
Against the smooth scrolling and slick HUD the enemy movement seems very clunky and gives them a cardboard cut out look, I think it may be due to using sprite animation rather than rotation for their movement...
Also the enemy just appear right in front of you rather than see them in the distance...gives a very cramped feel in a vast universe
Collision detection was a tad iffy and something that would probably bother Sci fi game fans (Your audience), is your shots hit exactly in the centre of the reticule, irrespective of movement, where in reality you would need to take the time for the shot to travel into consideration...