I've got one and I play it too.
It is difficult to make it all the way through level 16. You can earn an extra man every 40K, but you can only play 9 ships total. Make sense?
I think every 4th screen, it does something funky- when you have a certain number of crystal particles left, they converge on the center of the screen into a massive one that you have to keep blasting at. You have to destroy it all before a certain number of bounces or it will suck you in. Well, each time that screen comes around, you have less time to destroy the pieces.
Things can get pretty cluttered, especially on the waves where they keep sending the three different kinds of alien ships.
Here's one more curiosity- if you dodge the crystals long enough on each wave without shooting them, the level will end. It's pretty hard to do past the 2nd screen, but it works.
A friend of mine says that they used to gamble the lunch tab on it back in the day. It was one of the first games I bought AFTER I got into my house/garage and it hasn't been traded out. (too much crap in front of it now....)
Kerry