I have used the Circa by Levenger for several years and over the last couple of years have adapted it to GTD combined with Outlook. I used to use a PDA but have found the paper method to be faster & easier to scan. By utilizing the punch tool I can customize the agenda to confirm to GTD lists and I do not have to use the Circa provided paper. I really like the Circa products, and feel after the initial investment, it really is not that expensive.
FYI, in the past I have used DayTimers & Franklin Covey, Circa is the best based upon my experience.