Being doing it for years.. it helped me become a vegetarian 5 yrs ago after seeing what certain foods due to your body.
Closest thing that ever happened to me that was comparable to this master cleanse was being in a coma for a week and losing 30 pounds. When I woke I felt like another person and was sharper then I could recall.
Started today..
Pick up the book "Zero Limits" it will help you mentally get through this cleanse.