On one side, it's almost too many hours to waste for a free game.
On the other side, there are few things that make me bit unhappy about it.

First, few supposedly "easy" levels aren't easy at all and need some wild guessing.
Upd. Actually, just ignore all the difficulcy notes, every section has both easy and difficult puzzles. Challenge lovers should be amused.
Second, game would benefit from saving level progress, limitless undo and "reset level" button. And maybe a "hint" button as well. An UI/UX designer's touch.

It's 50/50 if I recommend it, frankly, and it's all due interface.