Frustrating, aggravating, and altogether not very fun.
As Dr. Salvadork said below me, it gets incredibly repetitious very quickly, however the main two beefs that I have with the game could be very easily fixed.
1) choose a different color palette. Half the time I couldn't even tell what color I was firing, and as such is far more frustrating than fun. Two blues? Come on!
2) Don't let the crystals move while you're shooting. I can't tell you how many times I wanted to throw something at my monitor because I was about to make the finishing shot, only to have an entire wall of crystals push themselves in front of the white gem... AGAIN... thus missing the shot and then have to spend another minute or so slowly chipping away at it. That isn't fun gameplay, that's just frustrating.
Another smaller beef is the lack of musical variety. This would have lowered the repetitiveness as well as kept interest a little better.