Unique, original, challenging, & addicting.
So simple in its concept yet so challenging to solve. It's not a puzzle where once you figure out a solution it becomes easy and therefore boring. Even once you get the hang of it, it remains challenging because of the ever increasing levels of difficulty. I like that you can keep trying to solve the same puzzle even after you mess up simply by hitting the "undo" logo or just give up and "rebuild" a new puzzle at the same level of difficulty. I also like that there's a timer that lets you see how long you took so that you can see if you're improving each time you play or how long it takes you to lose your mind. Kudos to the author.