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.
When I started eating mostly vegetarian a few months ago (by accident, I just didn't eat meat for a couple of days) I noticed my energy level shooting up, and I had the good (energetic) kind of hunger as dinnertime approached.
When I briefly tried low-carb breakfast and lunch last week, I got the same effect.
Is there an established theory that could explain this? The only thing I can think of is a crackpot-sounding book I once read that said you shouldn't mix carbs with protein since they're hard to digest together.
I've just read the Four-Hour Body by Timothy Ferriss. It seems on the face of it like ridiculously valuable material, if true - like what the completed version of Michael Vassar's proposed reboot of dietary science would look like at the finish point if dieting turned out to be more susceptible to Munchkinism than in my wildest dreams. Ferriss also talks the rationalist talk quite well in this book, much more so than in Four-Hour Workweek; he cites the experiments and occasionally says things like "I spent a lot of money on this and I expected it to work and it didn't work at all" or "I tried this and it seemed to work and I have no idea why it worked and I think it was probably a placebo effect."
Does the LessWrong hivemind have an opinion about 4HB? Has anyone tried it and found that it doesn't work, or that it does work, or that it works but not as well as Ferriss thinks it should work?