I taught "information design" at a major University for over 10 years, which means I teach others how to teach or instruct in various media. Before this, I spent 16 years designing computer software interfaces, web sites, and instructional material, including dozens of national and international awards for my work. As such, I am keenly aware of both the theoretical and practical issues and difficulties in designing instructional games for children.
The best-known, most-respected, and most-successful educators tell us that education for children (and for adults, for that matter) is most effective when it is fun, when it involves guided self-discovery, when it transparently and automatically adjusts the level of difficulty to match the learner's skills, and when it gives immediate feedback, especially when the emphasis is on positive feedback to the extent possible. Zoombinis excells in all of these areas.
My 9-year-old son has become engrossed by the game, and now spends time helping his 6-year-old sister. They LOVE the silly aspects of the Zoombinis ("hip, hip, zoombini!!!"), and are completely unaware that they are learning some rather complex logic and mathematical concepts. I was amazed when I overheard my son explaining to his sister how to solve the logic puzzles.
I tried playing it myself, and found that the animation and characters are not annoying, cloy, or childish, but possess just the right amount of levity to keep it interesting. I've found that after just a few hours of playing it, my son is already faster than me at figuring out the puzzles. He is now doing it at an intuitive level, while I still have to think it through step-by-step.
Unlike other software, which usually follows a fairly simplistic, linear algorithm that is easily defeated by simple trial-and-error (click everything until it works, then memorize the right pattern), the Zoombini puzzles/problems require the learner to internalize generalizable, transferable problem-solving methods.
Sorry if this sounds like mumbo-jumbo. The bottom line is that this is an outstanding educational product that teaches remarkably complex thinking skills to children while seeming to be nothing more than a really fun computer game. Buy it! You'll be happy you did.