I'm about to read it. Based on my knowledge of the guy, I expect the importance and expected efficacy of his findings to be exaggerated. But I definitely consider him worth listening to. I don't think he's consciously trying to con anyone.

A general caution about concluding that a new diet regime works: no matter what the change, if it involves food you don't normally eat, it's common to eat less calories initially; then you learn to like eating whatever the new thing is (possibly because of flavor-calorie learning - not sure how well proven that hypothesis is - I first heard of it from Seth Roberts) and are back to normal eating. There may be other placebo related reasons that support a new diet working only initially.

Also, some diets change water retention (higher salt -> more water held in your body), or amount of undigested mass passing through the GI tract, maybe in a good way - but changes in water weight won't continue indefinitely.