As far as I know, Optifast is medically supervised while Medifast isn't. Since you can experience some serious health problems by dropping down that low, it's recommended that you are medically supervised. We have several members here that have done Medifast, with varying results. Check the forum search engine for some interesting posts. One of the pitfalls with this type of diet is that it's so easy to regain weight (and faster) than other types of diets. It might be ok for someone with just a few pounds to lose, but if you have to lose a lot and need to stay on the diet a long time, then you may have problems maintaining. Look what happened to Oprah
If you do decide to follow this type of diet, please check the forum for others that are doing it and maybe you can find a diet buddy.
Good luck