I took the MGMAT course and I was glad I did. It helped me boost my score from the high 600's into the low 700's.
Having said that, I actually won the course so if I had paid out of my pocket, I probably would have been a little disappointed. Not a bash on MGMAT by any means, but I already had some of the MGMAT guides
and there wasn't a great amount of new content in the course. The instructors were phenomenal though and considering I didn't pay a dime, it met my needs.
If you have all the MGMAT guides
and you're self-disciplined to study on your own then I would skip the MGMAT course. I can't really speak to any courses from other companies, but I was disappointed with the Kaplan Premier
book - it turned me off Kaplan