Disclaimer: Before I write my review, I'd like to say that I am in no way affiliated with Manhattan GMAT Prep.
In the fall of 2011, I signed up for the 9-week, live online MGMAT course after several recommendations from friends who had previously taken the course and used the MGMAT materials. Although I had scored pretty well on the SATs back in high school, I knew that the GMATs were a completely different beast. Thus, I knew it was crucial for me to commit to rigorous preparation for the GMAT exam - and Manhattan GMAT prep provided just that.
In my opinion, MGMAT is truly the best GMAT prep source out there. I was incredibly pleased with how comprehensive the course is and how great the test prep materials are. I was particularly impressed by the following:
1) The strategy guides: these are super helpful, especially Sentence Correction (treat this guide as your bible - memorize it from cover to cover), Word Translations and Number Properties. The best thing about these guides is that they really drill GMAT concepts into your head, so that your understanding of such topics as prime numbers and permutations is rock solid. For example, I went into my preparation thinking, "I know my perfect squares!" but MGMAT materials really teach you how to tackle various problems in a very systematic and strategic way (i.e. perfect squares have an odd number of factors, whereas other integers have an even number of factors).
2) Official Guide Tracker - the OG tracker tracks all of the different problems one has completed in the Official Guide and categorizes the problems based on problem type, correct/incorrect and the amount of time one has spent on each respective problem. This is especially useful to gauge one's strengths and weaknesses. Using this application, I was really able to hone in on my problem areas and work at them so that I could succeed on the exam.
3) MGMAT CAT exams - I cannot emphasize how important these CAT exams are. One of the biggest challenges of the GMAT is staying physically and mentally alert throughout the entire ~4 hour marathon (during my first two CAT exams, I started to zone out around 30 minutes into the Quant section). These CAT exams are not only incredibly similar to the actual thing, but are also great for building up one's stamina for dealing with the long battle known as the GMAT. My only issue with these exams is that they tend to focus pretty heavily on combinatorics and probability-type questions; however, on the real thing (I took the GMAT twice) I have -at most- encountered 1 to 2 combinatorics/probability type questions. So don't stress out if you're bad at these aforementioned problem types!
4) General strategies/study methods taught in class - one of the best parts about the MGMAT course is the strategy taught by the (super helpful!) instructors in class. For example, I was advised to go through all of my practice problems -both the ones that I did correctly and the ones I did incorrectly- so that I could really grasp concepts and figure out quicker ways to do the problems I had already done correctly.
After my dismal diagnostic score of 590, I was almost convinced that I was never going to score above a 700. However, after going through tons and tons of timed practice problems, reviewing the MGMAT strategy guides from cover to cover (multiple times!), and stimulating real test-day conditions with the MGMAT CAT exams, I was shocked to find a score that was 180 points above my diagnostic (you do the math). I think that MGMAT was crucial to my GMAT success; however, one must also devote a good amount of time to really study the MGMAT material in order to succeed on the GMAT.