I've been meaning to write a review of this course (the in-person course, not the online one), for a while, so I guess this is a good thread to post it in.
In essence, the utility of this course boils down to a function of what your level currently is. I am stating the obvious here, but the
MGMAT class, well-structured as it might be, is not a magic pill that will help you cross 750. It best serves the purposes of those who have a problem understanding basics and drilling down fundamental concepts in math and grammar.
In terms of information about the test, there is nothing that is provided by the
MGMAT course that one cannot find on mba.com or elsewhere on the web; there is the advantage of having it nicely explained by the instructors, who by the way are all 750+ GMATters.
Course content is quite detailed, and once you've gone through all the classes, you know that you're not missing any core knowledge/skill that is needed. Here,
MGMAT shines with its well laid-out examples, and logical separation of topics. One is really better off taking this course before having worked with the OG, OG verbal supplement, and OG math supplement, because the course material is correctly centred on this material. I have not attended any other courses, so there's not much I can offer in terms of comparision with other 3rd party test prep material.
I joined this course mainly for verbal (started at 32-33 and got a 41 finally. I'm not a native speaker, and I don't read much non-technical material.) To be fair, I would attribute some of this improvement to
MGMAT, but its difficult to quantify, because I was working hard on other material like the verbal forum here (which BTW, is awesome) and the LSAT Prep books. The book on SC is pretty decent, and should be renamed 'SC for dummies' because of its easy style and logical flow. The RC and CR tips are average, and I did not end up using the
MGMAT approaches for either of these sections. However, I'd still recommend trying them out, you never know what works best till you've exhausted all the options

I know rhyme liked the RC approach.
For math, the books help to clarify all the basics you will be tested on.
MGMAT will not get you beyond the high 40s by itself. I personally found the math concepts covered poorly in the books, and sometimes confusing, because some concepts were explained in cumbersome ways. Importantly, none of these concepts are actually taught in class. You are expected to do your homework and clarify doubts in class. Further, none of the really difficult problems were ever attacked in class. I don't think I learnt much here, unfortunately.
One value-add at
MGMAT is the 1/2 hr slot per week that they allocate to each student. You choose a time and the instructor calls you up to discuss any problem areas or even specific problems you encounter. I think this was pretty good. The instructors are top-notch, and if you take this class, I'd recommend a lot of interaction during or after class. Certainly one of the high points for me.
Hope this helps...