Great Game!!
Man, the skull at the end was freakin' crazy... and freaky. Makes me wonder what the people of Earth are thinking...
I've seen the controls setup where the gun is independent of movement, but nevertheless it's put to good use in this game. Unfortunately, it's largely due to the fact that the enemy moves in somewhat random patterns. I'm not sure if this is classic arcade element coming into play or anything, but it sure pissed the hell out of me everytime I was forced to crash. Oh well.
Weaponry got a little hectic at the end, especially when the slice laser and the rapid lasers were exchanged frequently. Also, what seemed like an inconsistency is how weapons can achieve firing speed greater than that of power ups when you're trying to destroy an enemy at point-blank range. Great for pwnage, but otherwise it just doesn't seem appropriate.
Is there really a purpose to the gems? They seem to be bonus-only elements and occasionally can be found during a brief stage.
Even if there was an original game whose qualities you are trying to mimic, I felt that there could be some better work done at the "sound-like" music you can hear in the beginning. Great retro music nearing Saturn and Mars otherwise.
Overall however, it's a great and challenging game. Finished in 2280 seconds (37 minutes), and good quality time too!