Pros
With a very good organization and competent personnel, FOM provides plenty of opportunities to acquire useful skills and have more solid basis for the future career (courses, orientation, ...).
Yearly meetings are organized to allow all researchers to share their results and do networking, excellent working conditions, real interest for your personal path.
Cons
Difficult to determine. In some circumstances there can be some overlap between requirements from FOM and graduate schools, especially in the courses to follow, but it is not a real problem.