A little preface, I took Manhattan self study a few years ago and sat for the GMAT. I scored in the low 600's.
After contemplating business school, I decided the best chance of my acceptance was to retake the GMAT. After reading numerous reviews online, I decided Veritas Prep was the best choice, especially since they had a money-back guarantee if I didnt beat my previous live GMAT score. The VP materials were excellent and thorough, I think each section clearly broke down the GMAT subjects and strategies needed to master the test. For me personally, an in-person class was very helpful to grasp concepts not easily understood. Our instructor was very good at relating the material to practical business applications( ie: sentence correction errors to real life lawsuits where companies lost millions on the use of the wrong verb conjugation) and had great personal examples (catch phrases, stories, etc.) that allowed me to easily remember material when sitting down and taking the test. My biggest complaints were the classroom and pace of the course. The classroom was small, had tiny tables, was in a mediocre hotel, and constantly smelled of burned plastic. The pace of each night of class was very inconsistent. Often times we would go slow at first, then speed through the end sections(more difficult problems) and not finish each book. We were so slow that we had to add an extra night just to finish all the material, granted it turned out to be a nice review session of all the subjects. Don't think if you go to class, you'll ace the test. Outside of the classroom hours, be prepared to dedicate each weekend to 1 practice test and homework (8 hours at least).