I like Dr. Phil. I've got a couple of his books, and his common-sense "It-hurts-when-you-do-that-so-don't-do-that" advice is always funny.
But I agree, a fat guy who has no real medical background writing a diet book is questionable.
On the other hand, there was a profile of the Body-For-Life guy in this month's outside magazine, and he's a real asshole who first made a name for himself writing a book on how to abuse steroids.
As a general rule, I think most people take advice from people that are horribly unqualified to give it.