When I first started GTD I got into the whole list management apps, and new, cool tools that I could use with GTD, jumping from one shiny new object to another.
But after awhile I found I wasn't using them and I was getting behind on keeping my lists up to date.
So I had to go back to paper, and focus on the fundementals of the GTD principles.
Then once I got those down as habits, and really understood most of them, then I made the move back to digital.
Then I dropped my Iphone last year and sent it to a repair shop.
Well when it was at the repair shop the guy thought he would do me a favor and jailbreak my phone, and I lost all my data.
So I had to start all over again with paper and because I was scared of losing my data again, I stayed with only paper for about 6 months
But within the last 2 months I have started using a hybrid of paper and digital.
I use a Google Calendar, which I like because I can access it anywhere.
And for my lists I have the Things IPhone app, but I only insert 2 weeks worth of info into it.
For any actions or projects that I have that are 3 weeks or longer away I keep them paper based, and then once a week I add one weeks worth into my Things app.
I find that this helps me feel secure in case I lose data again.
And obviously like most others I have some reference on my laptop and some paper based. Same with my tickler files, project support files, and someday/maybe files.