I love this game. It's a real simple concept but very well implemented. One of its advantages is the fact that this game does not impose any stress. You can take your time and you always have the good feeling of doing something that actually trains your brain instead of numbing you.
Awesome game! I really enjoyed it. I took me several hours to go through all the 100 levels. It personally found problems 43 and 71 very difficult relatively to the others, and it only took me one hour to finish the last 29 problems. So I wouldn't say the difficulty is strictly incremental, as stated in the game description; but it is subjective and can also be a matter of luck. I definitely recommend it to anyone who's interested in visualization, planning or logic.