I only started using GTD this year. He has some brilliantly simple ideas hidden in an awfully boring book. What got me into it was the great system to plan. Instead of improvising planning and remembering everything you need to do, take one moment of the week to plan for the next week(s) so you can forget about what you need to do until you need to do it. I started using Asana for school, making a project for every course I had and putting all the assignments, readings & finals in there and plan milestones for them. It's such a great feeling not having to think about what I need to do when. The peace of mind is better than the productivity gains, I think. Now I use Todoist for everything. All my todo's have a due date and it allows me to keep track and focus easily. It's weird, I am much more motivated to start when I put something in there with a due date. I plowed through 14 todo's today, most of which I could have done any time in the next week.
Something Ryan Holiday mentioned in an otherwise forgettable and glad-handing book is that every night, he writes down what he's going to accomplish the next day in normal-sized letters on a 3x5 card. He said he finds that it's just enough space for a reasonable amount of goal-setting. My schedule has become so externally-driven that there's absolutely zero point for me to plan anything more than appointments.