I love this kind of puzzle, and games like this are the reason why. It was so relaxing and at the same time engaging, it felt like no time at all had passed when I finally finished it. The game is great, the execution is great, but... Oh, there's always a but... I didn't actually feel challenged in any level of the game, they were too easy! Maybe you could add some harder levels, I don't know... Or some specific challenges, for example, there were a couple of levels for which I didn't use all the balls, an acknowledgement of such accomplishes would be nice (so that you feel challenged throughout the game).
That's it, you get a 4.5! Please overlook possible english mistakes, I'm learning. ;)