Tim Ferriss is legit. The marketing speed is definitely strong, but there's substance behind it. His four-hour workweek, for example, was showed me how to create a successful business I enjoy (and work more than four hours on).

I don't like the slow carb diet personally, but it's better than what most people are eating, which I think was his goal.

The four hour body is meant to be read in parts, I wouldn't read it cover to cover. Read it with an open mind. If there's something you disagree with or wouldn't want to do, ask why he recommended it, rather than trying to prove it wrong. (Though do try to prove stuff wrong....just not as a knee jerk reaction)