The game is a simple physics puzzle game that is challenging and fun. I remember playing this game and its prequels on websites, so I have a personal nostalgia for it. And, to be perfectly honest, it's one of the better iOS ports of a Flash game on the app store--not for the game itself, but for how the port was handled.
For free, you get all the levels from the original Catch the Candy. The game does give you ads and punish you for turning off your internet (by adding timers before you can play certain levels), but the ads only appear after you've beaten a level (not each time, by the way, so it's more tolerable) and the game doesn't stop you from playing the game entirely if you are disconnected. Yes, there are games that do this specifically to make you sit through their ads, namely from the Cut the Rope series.