I purchased the full set of Manhattan GMAT books
on the recommendation of a friend. The set consists of 5 math workbooks and 2 verbal workbooks. The workbooks each contain around 8 chapters drilling down into various topics. Each chapter has a problem set of 15 non-OG questions that reinforce what you just learned. The workbooks also include OG problem sets. It gives you a pretty good workout.
I've gone through all the math workbooks, and I am just finishing the verbal workbook on sentence correction. My experience has been quite good, particularly with the math books. As someone mentioned, the math is at the higher end of the scale. If you need to review the fundamentals, start with one of the Kaplan
or PR books.
I recently took the GMAT but ended up canceling my score, and I have found that quite a few of the example problems in the workbooks are similar to the actual problems I encountered on the test. In fact, I was shocked to find one that was exactly the same.
The entire set is expensive (around $150), but I think it's worth the price. If I had used these my first go around, I wouldn't have had to cancel my score (at a price of $250). Some GMAT prep books are likely to be the cheapest part of your MBA experience.
Hope this helps!