They teach the seminar in New York and I am based in California, so I have never seen it. One of my students has taken it and liked it. She said it was worth the money. That is all I can tell you about the course.
I can tell you that the company and instructors are certainly qualified to teach such a course. MGMAT publishes a "challenge" question every week (i write some of them) that is much more difficult that those you will typically see on a GMAT, but about as difficult as the hardest questions that might pop up for a true 750+ candidate. Each question is carefully review by the boss to make sure that it is appropriate, and that the explanations are clear and concise. It might be worth your while to subscribe to the archives (you get access to almost 2 years of qustions).
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993