I have been to both companies' free info sessions. Both have great presenters. However, Manhattan Review offers more than just the GMAT, they also do free MBA admissions evaluation. So one bird kills two stones for me as I want someone to guide me through the process after the GMAT. My verbal instructor Mark was terrific. He allows us to ask any questions we have. Our entire class was enamored with him. I actually recommended a friend to his class at Manhattan Review's Grand Central location. They don't seem to be active on any public discussion boards while
Manhattan GMAT does have a presence. One thing we should all bear in mind is that how many of those posts are bona fide. I read on business week site that
Manhattan GMAT was posting "love..love..love the class" type of comments on an article about the GMAT prep companies. Someone called them out immediately as it was absolutely fake.
I heard great things about some other companies such as Knewton's online courses, etc. My advice will be doing your own homework and find who works out best for you. All the fluff on the net is good extra info, but we need to take all those "unaudited" praises or complaints with a grain of salt.

Best luck to all of us in this endeavor and also in the arduous job searching process.