I really bought into GTD a few years ago when I had become overwhelmed by my to-do list. I found a really good book on implementing GTD within Outlook, which suited me as I use Outlook with Office 365 on a daily basis.
I read all about TSW and attempted to implement it in Evernote. However, as well as being rather complex, it felt very unnatural to be saving emails into evernote rather than staying in the place I was already using - i.e. Office 365.
However, I really liked the Now, Next, Soon, etc categories. So now I have modified my Outlook GTD implementation to incorporate the 'w