I remember seeing an infomercial on that 15+ years ago. I think it is super cool! I don't think it would cause any harm and I believe it would be fun for many kids.
I found the book @ our annual library book sale & snatched it up for a couple dollars. It's not bad. Although it'd be a beast of a way to teach math outright.
With our son, I just left it out "accidentally" for him to discover on his own (otherwise it would be *forced* learning!). He saw some cool tricks, but I told him he had to learn the "right" way first so he could show his work on tests, etc. Afterward, he's free to learn & use whatever trick he wants.
I like the one that lets him give your the DOW for any date, or something like that. Merely a parlor trick, I know, but still pretty cool.
These "tricks" are also useful for double-checking answers, which can be tedious otherwise.
While the things he learns won't always be applicable, he at least adds some tools to his problem-solving arsenal. And so far he's pretty good about choosing the best approach for a given problem.