I don't know for how many years I've received Levenger catalogs featuring their Circa notebooks yet never paid any attention to them until reading posts about Circa on this form. It is an interesting concept. Like a spiral-bound notebook--but unlike a ring binder--the Circas can be opened and folded all the way around to lay flat. And like a traditional ring binder--yet unlike a spiral-bound notebook--the pages can be removed, re-inserted and re-organized.
A few years ago, David Allen wrote about what he called--if I remember correctly--his new universal capture tool; a rather smallish spiral bound note book (approximately 4x6 ??) with perforated pages. More expansive than his notetaker wallet but small enough for easy portability. Pages could be torn out and placed in the inbox.
Because the pages can be removed and re-inserted, the Circa looks like it might be quite useful for project notes, especially those taken away from the desk. And the refills appear to be cheap enough to be used like Allen's old universal capture tool.