A must have!
I couldn't have prepped for the GMAT without this guide. It is a "must have" for anyone who actually wants to study for the GMAT with real GMAT questions. Kaplan's questions are too hard, Barron's are just bizarre - you can rely on these questions.

not a must have but good for practice
most of the stuff in this book is already in the GMAT official guide(OG 11) except the practice problems. buy this book if you want more math practice...it has good amount of practice problems: Problem Solving(176), Data Sufficiency(118)
and these problems are different from the ones in OG 11.

Excellent Preparation Book for the GMAT
It has been seven years since I have been out of college. My math skills are very rusty. The Quanative Review is the best book to get because it is written by the GMAT Staff. Each question comes with an explanation of how the problem is solved. The book is broken down into sections with complete explanations.

I am not good at math at all and there are over 300 problems to solve from basic arthimatic, algebra, geometry, and Data Sufficienty. all have multiple choice answers and at the end of each section.

The book is much cheaper than getting it through the mba.

I also recommend the GMAT Verbal Review as well

Helpfull for GMAT excercies
This book is a must if you are about to take the GMAT and need lots of practice on Math it contains a lot of good exercies and how to solve them...

GMAT Books you must have!
I have been studying for GMAT for a few weeks now and I think the OG is a great source for a good practice on real GMAT questions. But having said that, use the OG only once you grasp all the basics and when you are almost ready to take the GMAT. The OG does not contain any math conceptual content, so use it only for practice towards the end.

In order to get a real GMAT prep - I would recommend you buy the set of 9 books of "EZ Solutions," or whichever book(s) you think you need most help with. These books are very detailed oriented and cover everything on the GMAT math. First use the review modules to get the concepts and then use the workbooks for practice. Some of the most challenging topics in GMAT math, such as permutation/combination, probability, geometry problems, complex word problems, etc., are brilliantly explained in these books. Also, the EZ Advanced Workbook is a "must have" if you are aiming for a high score. It may be a good idea to buy Barron's without the CD (don't waste money on the CD version for any book). After doing all this, use the OG and you will see a dramatic difference in your scores. If you still need more practice, you may consider buying the Kaplan book (but really not needed). Save your money by not buying anything else. You don't even have to take any of those pricy courses. I followed this process and my scores have jumped from the 500-range to the 700-range. Good luck!