The Master Cleanse is a fast. It's really more like a juice fast - kind-of. You can read all kinds of testimonies in this forum's archives. I also recommend reading the FAQ's and the recommended messages, at the top of the page.
If you don't have a copy of the book, google "Master Cleanse pdf" and read the online copy.
The Master Cleanse is really a good thing. Study up on it and then give it a try!