However, I have never been able to find a good implementation of the system. I know that David Allen (GTD) and Charles Simonyi (Intentional Software) were working on something at some point, but I haven't heard anything since.
Like another answerer to this comment, I finally rolled a custom Sheets layout that works the way I think about things, and over the last few months, have slowly tweaked it closer and closer to my personal thoughtflow.
It's not the details of the system that matter, it's the relieving of the stress that makes it worthwhile.
1. ThinkingRock
It's a desktop client for all operating systems. There are two versions. A free one (v 2.x - still available on sourceforge [0]) and one you have to buy (v 3.x)[1]
2. Getting Things Gnome
I find it slightly less convincing than ThinkingRock. What's more, it appears not to be maintained anymore. The offical website is down and there have been no update on github[2] for a while. However, it might be just what you're looking for so you should also give it a try.
Disclaimer: I work at Zenkit.
I have some issues with it (primarily that it's hard to share information from it with other people), but otherwise I find it to be a fantastic personal tool for GTD (I'd use it for work, but my office is all Windows).
That said, I do like paper and org-mode at least as much. I definitely find paper best for writing down my intentions and clarifying them before I enter them into OmniFocus.
- The context support is weak. It ideally would be replaced by a tagging system, but I doubt that's coming at this point.
- No support for making sequentially dependent tasks -- I can't mark task A to be doable as soon as task B completes. (I guess I could, if I do a lot of sub-projects with sequential dependency)
- In addition to this, subprojects are janky. They're just kinda awful to use, from the awkward indentation to the fact that the side-bar projects view is not actually a full tree, and just works with folders. This would need to be changed to also expand/be usable for sub-projects, not just for folders and top-level projects.
- The system maintenance tools are lacking. One of the core tenets of GTD is to make sure that every project always has a next step, and OmniFocus has no support for this whatsoever, aside from manual review. I realise that paper also doesn't do this, but it seems like something a digital solution should have.
- No sharing information, as you said, aside from exporting things as TaskPaper.
- Support for deferments, but not for Tickler files, which are one of the handiest things about a paper-based system.
- Reference and storage, per nature of OmniFocus, has to be completely separated, which adds jank. I realise this is massive scope-bloat, so it's probably not fair to complain about.
Things I really like and will probably keep even after I drop OmniFocus:
- The dedicated mail drop. I think even after I move away from OF, this is going to be something I keep.
- The "due" list view. OmniFocus pushes you towards using this a lot, even if it's not completely in the spirit.
- The good integration between my laptop and phone is going to be hard to live without again.
I hope this shed some light on my gripes and issues with OmniFocus. :)
Disclaimer: I work at http://zenkit.com
If you're just looking for a summary: there is very little point writing things in a list if you don't read the list, obvs.
The key to GTD is that if you actually do write everything in lists and calendars, then you are freed from having to remember things. If you can reach that inflection point, then it frees up a lot of mental capacity and is glorious. Remembering things is surprisingly mentally draining, as it basically consists of thinking about them every now and then.
Read the book.