I have a copy of this book

This book delivers what it promises. It'll be useful to you if have trouble multiplying, dividing, or need help finding square roots, or squares.

The approach in this book is good if you need help on working with large numbers. It's a real time-saver there but I haven't come across too many large numbers on the GMAT, and when I have they have mostly been solved by applying the right concepts.

Sometimes you do need to work with big numbers. Check out the link below

http://www.gmatclub.com/phpbb/viewtopic ... highlight=
Get this book if not being able to perform calculations is whats holding you back but before you buy the book I advice you to sample it here

http://mathforum.org/dr.math/faq/faq.trachten.html
Maybe it won't be a bad idea to try this in some of your practice tests:

Solve as many questions as you can and when you start to run out of time, pick any answer choices on the remaining questions to get to the end of the section. See how this strategy works for you.

BTW, the opening of the book tells a little about Jakow Tractenberg, the founder of this system. He came up with this system while he was in one of the worst Nazi concentration camps and Hitler himself gave orders to kill him so everyday was like his last. He had no paper to write on there and he came up with all the techniques explained in this book by working everything out in his head!

Pretty amazing, huh?