For me, this was one of the best toys ever invented. The concept is very simple, but the combinations run into the zillions, which means that you can't solve it by chance.
However, one can have fun working out how to solve it, then once you've done that, impress your mates by applying the same principles over and over again. There isn't that much to remember once you know how.
The main problem is in the mechanism and the stickers. These things don't last forever, and can start falling about once they've been played with quite a lot. Also, I have experienced the paint fading on some of the stickers. So it could be much more durable than it is.
However, this is definitely one of my all time favourite toys.