I have spent over 23 minutes writing this, wtf is wrong with me? :
In my own (slightly bizarre) experience, it's more about the habits than the "fancy checklist" system. It's more than a checklist system, more than a calendar, etc. It's how these pieces work in concert with one another.
Whether through GTD or some other system of operating/thinking, we are all craving a way to have things taken care of for us and not having to worry about them. Imagine if you had a little robot friend that reminded you about your appointments. It would wake you up when you wanted to be woken up (and never failed you like an alarm clock). You could tell the robot to remind you of an idea you just had and want to think about at a different time & the robot would remind you at a good time.
This robot could be seen as an extension of the mind. Like the invention of ink & paper.
But this little robot doesn't exist yet. We have to use our own hands and minds, or we can delegate to other people. GTD is an attempt to do this.
Now I should admit that I do not follow GTD fully. I am using GTD plus some other pieces, but I do see GTD as one of the hearts.
And this took me 2-3 years before I started to reap the benefits. For the first 2 years it was an off-and-on thing. Things would run smoothly, and then a clog would get caught in the piping, leading to productivity relapse.
The problem with GTD is that there are too many integral pieces, and if any one of those pieces breaks down the whole system breaks down. I was fortunate enough to have health issues and not have a job at the time so I could pour countless hours into this on the faith that I would reach consistency.
In hindsight, the only reason why I bought into GTD religiously in the first place, inspiring me to put in retarded amounts of time was that I was reading heavily into willpower, and the importance of conserving mental energy, especially for decision making and other high-level brain functions. The book Willpower by Roy F. Baumeister had endorsed David Allen and GTD.
I respect Baumeister immensely as he is an opponent of the self-esteem movement, a movement that has really fucked up many peoples' ability to self-regulate their emotions and focus. As everyone was focusing on the promises of higher self-esteem (Baumeister also started in this area), he was researching things like self-control, emotional intelligence, focus and willpower.