Not bad, but...
The bonus levels were pointless. Mostly just a place to die eithout earning any points. Some of the asteroids couldn't be destroyed, they were glitchya dn shots went through them. Why did the asteroids require gradient meshes on them exactly? Just to lag the game?
I'm against people complaining about lag when a game actually warrants a faster processor, but in this case there was no need for the use of resources.
The levels didn't change at all except for how the aliens looked. It was never any more difficult, at least not the first eleven levels I played before getting bored.
Also, I'm pretty sure I played this game before on my Atari back in the 80s. :P And that one actually did get harder with each progressive level, and didn't slow down any processors with pointless gradient meshes.