Overall, it is a great game. The music is well chosen and not distracting. The gameplay is fun and addictive, but I do have some issues with the graphics, perhaps it's just me.
I find the ship is a little static (not much animation) but I don't really mind.
The background is a little distracting though, with the fast flying asteroids, sometimes I confuse that from the actual enemies, end up firing a few rounds at it (not much penalty though). Perhaps change the color of the animated asteroids to something lighter? Then again, it's just me... maybe.
Another issue I came across is the lack of warnings.
Right before fighting the bosses, there are these -stalagmites?- that came in without warning. During the first encounter, I thought they could be shot, and tried but died after colliding into them. Second time, I found out there's a little gap that you can fly though to avoid, but that gap closes after the stalagmites reach the midway point of the screen. It punishes players that weren't daring or fast enough to move forward. I suggest editing the behavior of the stalagmites so that it gives a second chance to players who missed the first gap. At least their death is more justified.
The boss battle... the only thing I hate about it is the sudden appearance from the back. It's cheap and I felt that it was unfair that I should die to that. There is no warning whatsoever that the boss was coming from the back. The only visual warning that I had was the boss flying at the background, and he disappears to the right of the screen, which makes me automatically assumes that he will reappear from the right. Change that.
I suggest giving some warning or hints of something that can potentially kill players from behind, this way, players will feel that it is fair to die to the boss if they did not heed the warnings.
Overall, a well done game (especially since it's a demo). Interesting concept with the gun and shield, since many shoot-em-up games have you dodging the bullets or projectiles.
I'm not beating down on the game whatsoever, just giving an honest opinion.