Not only is it challenging, but also incredibly addictive. It's also great that there are SOOOO many puzzles to figure out, as well as achievements and unlockables to get. And as a person who likes to complete games 100%, the ability to solve a puzzle perfectly increases the replayability of the game even more.
So in conclusion, I think my overall opinion of the game is pretty much obvious: I ABSOLUTELY LOVE IT! I've already played it for over seven hours trying to perfectly complete all the easy puzzles, and I'm so ready to think out the rest. Very well done!