"I've been using Levenger's products for about 10 years, most notably their Circa notebooks. The ring-based binding allows not only quick removal and replacement of pages, but also a nice mix and match format for a variety of sheets, including ruled, blank, grid, address book, calendar, and more. I have a couple leather covers that I change around a bit to reduce wear on the notebooks. I tend to walk around with a normal junior size Circa notebook (about 5"x8") with pages of several types inside, including little contact information cards, 3x5s, etc. Sometimes I'll take a page out and offset it by one ring so it sticks out above the others to denote a new section (though I believe they have all kinds of dividers and such now). The rings themselves, though plastic, are sturdy and heavy -- notably better than the Rollabind notebook. The paper is a fairly heavy stock with good absorption and a great feel when writing on it with either rollerballs or fountain pens. The line printing is subtle but clear and it comes in a variety of layouts, though I tend to stick with the Columbia note taking system layout (with a column down the left-hand side), which used to be their only offering. They also sell paper in larger packs of 300, so I tend to keep a couple reams on hand. The hole puncher from Levenger is also a well-machined thing..."
Mike Wilson @ Kevin Kelly's Cool Tools