Music Catch isn't a game so much as it is a Flash relaxation device. The gameplay, such as it is, is very simple: a piano piece plays, and as it plays, shapes will appear from a continuously rotating line on screen. You move your mouse to catch the shapes with your own shape. Yellow shapes will increase your score multiplier and make your own shape larger, making it easier to catch more shapes both good and bad; red shapes will decrease the multiplier. Purple shapes activate "purple power", which draws all non-red shapes towards your shape for a limited amount of time. And, well, that's about it. You play for the length of the song, and then see what your score is.
The music is a pretty little piece, although the number of comments saying something like "I don't normally like classical music, but this is the best ever!" makes me despair a little, since there definitely is a lot better out there. Still, it's a lot better than your typical Flash game music. The graphics are pretty and colorful, albeit not particularly fancy. As a game, it's not perfect -- some times, you might get lucky and get a lot of yellows and purples early, which helps to bring up your overall score a lot. Also, while each yellow increases your multiplier by 1, a red can cut your total multiplier in half, which can be awfully frustrating, especially in the late game when your multiplier has gotten very large.
Anyway, that's mostly nitpicking; overall, this game is quite successful in providing a lovely interlude from the more stressful, action-packed games on Kongregate.