Hi there,
I studied this diet pretty extensively for our family, but specifically for my son. I know people who have personally used it as well (at least a handful). The diet does work to provide some gut healing. It can be an effective "stop gap" measure if one needs it. The one person I know that was successful with it for her autistic son took 7yrs to get significant healing. Dr. Campbell says 2yrs but that is somewhat unrealistic from what I've seen amongst friends.
I think her basis is absolutely correct. Disease begins in the gut and that must be healed for any other part of the body to heal. I disagree with her on the use (or better said, non-use) of probiotics during intro and parts of phase 2.
It is a HARD diet to follow and restricts life tremendously. Which is why, in the end, we decided not to do it. We were already gluten free, dairy free and 38 other foods free. Going to non-stop bone broth soups just did not do it for us. However, when Luke had no platelets and we were in wait mode for how we were going to treat him, we did use bone broths 6x's a day to feed him instead of just food. He had bone broth before he ate anything else. It filled him up, but we knew in the long run it was at least aiding his body for the moment until we could determine how we wanted to deal with his ITP. As soon as we went to h-pathy we put him on a normal diet. We eat a lot of bone broth in general every week so we were confident he was getting enough just in our normal diet. This is the type of "stop gap" I was referring to.
My experience over the past 5yrs has been that diet alone will not heal the body. It is a MUST to aid in healing, but it will not heal it by itself. I think the GAPS book is worth reading from an educational standpoint, but doing the diet will require full time commitment (forget it if you work outside the home or he goes to school).
My .02 worth.

I can give you more detail if you want, but that's the basics of what we saw and decided.
patti