I like your ship designs. And the use of distort on the hills is pretty slick looking... but it's kind of hard to tell what you can and can't fly over untill you hit something. Perhaps some sort of visual marker on your hills, like a texture of a treeline, or snow, or something like that to indicate that that part of the hill is too high to fly over.
Also, I noticed in a couple spots that I hit the hill well before even reaching it:
<img src="http://i50.tinypic.com/11glnwy.png">
I run into it at the red line, like the collision is off or something.
I agree with PR that the enemies are too hard... I died on the first one the first time I played it because the combat was weird. I can't turn my craft fast unless I'm moving? It's not a car . So here I am, turning slowly in circles, and the enemy ship is quite literally flying rings around me blowing me to shreds because it's just out of reach of my bullets. And they have too much health, really... enemies in games are generally a lot weaker/slower/stupider than the player, so the player can slaughter dozens before being taken down, so it's kind of an odd sensation to die on the very first enemy you see.
I did manage to kill them all on my second try by flying away from them, turning around, and letting them run into my bullets as they chased after me, but it didn't feel all that satisfying, and it got kind of old after a bit because they had so much health that it felt more like a chore than a dogfight.
Also, the sound of the gun is really too harsh, especially when holding it down for rapid-fire. I found it to be a bit grating on my nerves.
The AI on the enemy ships isn't bad, but it is kind of odd to see them always pointing directly at the player, especially when the player crosses over them and they flip around suddenly.
I am interested in seeing more, though, so please don't take my criticism to hard! I only want to see you make the best game you can