I'm a pretty faithful GTD user, and I love the book, because I need to hear some of those things a hundred times, but also because there's nuance to the method beyond a few mechanical basics. I've met a lot of people who say they do GTD and haven't read the book or only skimmed it, and invariably they seem to basically just use to-do lists. That's a good first step, but it's a terrible place to stop.