After completing the 60 day program in this book, I can say that there are both advantages and disadvantages to it's way of approaching the idea of 'brain training'.
On the one hand, since most of the book is concerned with daily mathematical equations, this is the predominant aspect you're meant to train. The significant drawback then is that while this may be of use to those needing help in this area, it does not necessarily do much for others.
I my opinion, the chief advantage of the book however, is that it forces you to develop a daily routine making time (and head space) for the exercises. I found that more than the ability to calculate (which over the 60 days did not really improve that much for me), the training to concentrate myself exclusively on the task did. While this is hinted at in the introduction to the book (which tends to focus more on explaining the different parts of the brain and their functions), I think it's actually one of the main goals aimed at by this system (and the reason I gave it an overall positive rating).
While I do recommend it, being a bookstore employee I feel obligated to add the caveat that this is an exercise book, and therefore will only give you results if you follow the program completely through. I do think you'll see results, but again, this is a book - not a miracle cure - so your expectations should be moderate.