I'm doing this mostly on my own. I buy my Medifast through Take Shape For Life, which is like a sister company. They set you up with a health coach who is supposed to help you out. I'm really lucky, my coach is an old friend, so I can tell or ask her ANYTHING. I find the online support really helpful. I don't even have a Medifast center nearby, so it was not an option for me. I work a crazy rotating schedule, so I don't think it would be convenient for me if it was available.
Do what you think will work best for you. If a weekly meeting helps keep you motivated and honest, then the money spent at the center may be well worth it.
Lost the first 100 with Medifast, now working on the last 25