I have used the EZ solution advanced math workbook. I would say its really good if you are already a 600-700 scorer. One thing to know about this book is that the questions in it are not like the GMAT question. Here you wont have 5 answer choices, you would just get a lot of topic specific (tough) question which you need to solve. This would definitely increase your math ability, when you start hitting the upper bin questions on the real test. Plus the explanations are great!!!