Pros:
It explains what Mental Diagrams are (a form of mind map) and why they are useful.
If you want to use them for a variety of applications - making notes, planning, presenting - then it does a good job walking you through the process. I liken it to using mnemonics to remember a deck of cards. You have to do a lot of prep work to ready yourself to be able to remember a deck quickly. Similarly the book walks you through the relaxation, meditation and alpha thinking techniques required for visualization, how to choose the right visuals, the importance of having short words in the diagram and so on. There is a lot of good stuff there in the exercises, which to me was worth the cost of the book in itself.
Cons:
This book appears dated. For example it refers to overhead projector slides, which I have not seen used in a corporate setup in the US in this century. Also there is a lot of focus on manually creating these diagrams, which is cool and all, but almost everyone puts their notes online now. So if the book was relatively recent I would have expected some reference to mind mapping or other diagramming software, which is sadly absent.
As other reviewers have mentioned, some of the science we know is incorrect, some just feels too weird to be right. But the point of the "science" is to help explain something else, so I'd give it a pass on that front.
This book appears to have been a translation from a Spanish original, which explains some of the language. But I read it more as a reference/textbook and the prose is thus not an issue.
Overall, I don't regret the time I spent reading this book. It certainly helps expand your mind to new techniques of memorization.