I have yet to write up a formal post on it, but I have my (letter) Circa divided as follows, from front to back:

* 4-5 sheets of blank paper to take quick notes. All I need to do is open the cover.

* My calendar/agenda. I use the DIY-planner templates. I have a weekly format for the next six weeks followed by monthly for the rest of the year for long-term planning.

* Projects. This section is where the Circa rings excel. I don't keep a "projects list" per se, but this section contains one piece of paper per project, outlining the necessary actions. I use different size paper (index cards, classic, and letter) depending on the size of the project. Not only does this save paper, but it's also a visual queue for me to know how involved the project is, bigger chunck of paper real estate = more involved commitment. I can spread my projects out on a table and it has the appearance of a lo-fi "tag cloud". Very handy. One thing of note with my classic sized pages: I cut them from standard letter sheets. Rather than punch vertically, I punch along the 5.5" size. This allows me to save space in my notebook by placing 2 classic pages together and inserting it into the notebook as though they were one letter-sized sheet. It's not much, but I prefer a thinner notebook.

* Project support. I keep this separate from my project planning. This section contains any notes, images, Evernote printouts, documents, and anything else I might need for any of my active projects.

* Address book. I'm in Windows, and I manage most of my electronic reference material in Evernote, addresses included. This section is a printout of my addresses, updated as needed.

* Notes. Contains blank paper of various types, blank, ruled, and graph. I find I tend to be using blank more often lately. I'm using HP's 28# laser paper, which shows great durability for the circa and holds fountain pen ink very well. From what I can tell, it's slightly thicker and a lot smoother than Levenger's paper, at about 1/10th the cost.

As a sidenote, my Circa doesn't contain my entire GTD or reference system, just the things I need to have with me while away from the home or office. I keep my N/A's in my shirt pocket briefcase and someday/maybe items in a 3x5 card box at home. I don't keep an "inbox" section in the Circa either. Instead, I have a bright red folder in my briefcase. I simply take the unprocessed pages out of the circa and file them here. They can always go back into the main Circa, or one of my other reference Circas (which, BTW, include work projects, class notes, reading journal, or anything else I might need).

Anyway, I hope this helps. I went into more detail than I thought I would, but that's okay :) So much for not having a formal post anymore!