I weirdly almost envy people just starting out with GTD, as although I found it tine consuming in the beginning, to get my head around and to try out varying approaches to implementation - the first few "wins" for me made such a positive impact on my sebse of control and organisation, I wish I could go back and have that high again. ( okay I know this sounds evangelistic but It isn't really, because you have to stay in control of what you want out of it and tweak it to meet your needs all the way along - if you are the master of GTD rather than the servant then you will see the benefits too.)
I started with the book and with notebooks and after three years my expenditure gas been little more than this. It will be a different journey for each person it for me the immediate wins were around dealing with "stuff" and increasing productivity around next actions; propelling projects forward at work. But it took longer for me to really engage the vision, purpose and areas of focus to work and this is what I am making progress on now and that feels really good. It's a big system which can feel extremly complex and extremeky simplistic at varying times in tge learning process but then life can often feel the sane to me - it's this synergy that tells me GTD has merit.