After trying GTD for about 7-8 years, I gave up.
I mean, I kept using it, thinking it was working, but I'd look back and ask myself key questions:
1. How often do I stop looking at my lists, because I felt overwhelmed?
2. How often do I need to spend a large amount of time cleaning up the lists?
3. How often are things getting missed? How often am I doing things not in my GTD lists because I couldn't figure out how to put it in there?
4. How often do I tweak my GTD system to fix the above?
And so on - I realized that while GTD was of some help, it was not really working.
It did have some useful things/ideas, and as such it was not at all a waste. However, it really didn't do a good job of the fact that my lists were huge. I think he recommends looking at your Someday/Maybe in the weekly (or monthly?) review. That list is huge.
Even the TODO list can be large with his system. I don't think he addresses granularity well. Should my TODOs be the mundane small things, or just the big picture project (he leans towards the former). In reality, the potential Next Action on a project could be multiple things, so I would have multiple TODOs (it's not always clear which one I can do first due to external constraints).
His system is mostly priority agnostic. He does address it a little (10000 ft view, etc), but it was very vague.
No clear guidance on how to know if you're trying to do too much. Especially needed with GTD, because as a system, it makes it easy to try to do too much.
I think if someone could write a book with all the stuff GTD is poor at or doesn't address, with solutions, then GTD + that system may actually be great.
It's a good book, but don't beat yourself up if it doesn't work well for you. Try to tweak it to your needs, and if that doesn't work, look for something else.