First of all, the graphics and music are just wonderful, and perfect for each other. I was really impressed by the way the block's sides even move like they're breathing as they sleep. The puzzle is simple and easily mastered, but hard enough to require thinking. A great game!
.... until you get into the later levels. Then, all skill, all thought, all everything goes flying out the window, and it all comes down to getting the right color at the right time. Okay, I can understand that. However, by the end, it was getting ridiculous. I kept getting the same color over and over, and it was always for a block at the bottom of the pile. So I would finally find a block of that color to remove without stirring up the pot too much, select it, wait for all the whining to die down ... only to get the exact same color again. VERY frustrating! I've tried to get through Level 14 probably two dozen times, before just saying "screw this" and moving on.
It might be better if you made it so that you couldn't have both cards be the same color, or made some sort of switching system (where you can "reject" a card so many times per level). Or even have a few wild cards in there, blank cards that allow you to pick any color you want. Something like that. Otherwise, it goes from "challenging puzzle" to "impossible luck-of-the-draw gamble" towards the end, and isn't fun for anyone. Except the gamblers, who probably shouldn't be playing a puzzle game, anyway.