My experience with GTD is: trying, failing, trying again, failing again, etc. So I wouldn't say it works for me. But I have noticed that waiting for a perfect implementation is a good excuse to procrastinate. And there is no guarantee the perfect implementation would make things work. Or I could change my mind and decide I now have different criteria for a perfect implementation. Etc. So it's probably just an excuse. I could start with what I have now, and it does not prevent me to search for a better implementation.

And probably I am using it for much simpler tasks. Because my typical situation is simple tasks not being done. I can still imagine that for a complex project I would use a Word document in computer, and paper for "next actions"... because the "next actions" by definition are simple.

How exactly did you fail in your previous attempts with GTD? My typical failure mode is simply giving up and ignoring the system. The same thing could happen to me with a computer implementation, too. Some problems have to be fixed outside of GTD. GTD is basicly for people who have no problems with procrastination, they just do tasks in a wrong order (less important ones first) or remember them in wrong contexts (e.g. forget to send an important e-mail when at computer, remember it when walking outside, then forget it again).