So, I ground my way through the entire game. One star for being addictive. Another star for the graphics, they are very suitable for this genre of game.
But... that is it. The final 70 or so levels were exactly the same - other than adding the higher level alien guns to my ship as I collected them, I changed absolutely nothing on my ship for the majority of the game. Ended up finishing with 130,000 crystals with nothing to spend them on.
Just buy shields, lasers and use call for help when you get to the boss.
I don't recommend playing all the way through - you get nothing out of it. The end is "Thanks for saving the galaxy" and *bang* all progress is lost.