Because of that book, within 3 months I went from running completely out of breath after 2 minutes of running, to finishing a half-marathon in 2 hours. And during the prior 3 months, I had lost 15 kilos by following the "slow-carb diet" described in the book.
Reading it seemed to flip a switch in my brain: before, I would think of my body as something I had little control over, while after, I saw it as not only something I had full control over, but as something I could hack. I've also followed up on quite a few of the product recommendations in the book (e.g. Inov-8 trainers, Aqua Sphere goggles, etc), and have yet to be disappointed.
That said, the book does come with a heavy dose of Tim's pointless boasting, half-assed chapters (e.g. the polyphasic sleep or the baseball batting ones), and far more conjecture than a book of that sort should have.