I've been half following the GTD method for years with varying degrees of commitment (I always thought I knew better than the experts... silly me). I've bounced back and forth between digital and paper... keeping everything synchronized has always been my Achilles heel.
Recently I decided to give the system my full attention and really take heed to the expert advice available. Signed up for GTD>Connect and was set out to build a full-size paper based system when simultaneously I realized this was going to be far too bulky for my use and how the "notebook" format is ideally suited for Microsoft OneNote. Sync issues solved with the OneNote Android app and by nature of the beast should I decided to go back to paper I can simply print out my OneNote, punch and insert into a binder.
I still love paper in my heart of hearts and to satisfy that a "reporter" Moleskine notebook and Pilot G-2 pen are always in my pocket ready to capture quick thoughts and inspirations and an A5 Moleskine stays based at my home office for end of day reflection and journaling.