Incredibly fun.
This is pure genius. Great concept, great game.
Yea, it's really difficult, but it develops your "thinking-outside-the-box" skills.
It's probably easier for people who are very visual in their thinking processes, so most of the people complaining here simply have trouble visualizing (in their heads or on paper).
So all in all, great game. Very fun. Very addictive. I'm on level 61 now, but I've gotta get some sleep. So I'll continue playing again tomorrow.
The world needs more good games like this.
(By the way, you should make a note that some of the answers are two words, but with no space in between.) I only remember one level with such an instance, but still, some people might get stuck.