I have the free levels, but haven't upgraded to the paid version yet. I'm not sure if I will, as this seems to be an exercise in frustration.
The graphics are nice, and the background music is appropriate. The sounds of the balls, boxes, etc are all very well done, and sound great with headphones.
I think the problem I have is that the game does a poor job of settling you in to the concept. There is a tutorial button, but it leads to a blank page with a "done" button, so that's useless. The Issac Newton in game character provides some help, but it's very limited and doesn't offer any help if you're stuck.
The first level alone had me baffled for a while. Once I figured it out, I was like "doh!" But now the second level has me stumped... I feel like I shouldn't be stumped until at least level 4 or 5...