I have used them for about a year now. I used to use the pocket size but its too small for diagrams / mindmaps / long lists.
I use a large one, just a bit bigger than a paperback (13cm by 21cm), for GTD stuff and detailed work notes but I find I still keep a pocket one (9cm by 14cm) handy for creative writing ideas ( just as well as I bought a few of them when I could ).
They come with an elasticated band to keep the book shut, a placemark ribbon and a concertina(accordion) pocket inside the back cover for those scraps of paper - I keep an unread magazine article in there for when I have a moment to read something - waiting for a train.
By the way they come with either plain, lined or squared paper. I have the squared ones as it makes indented lists clearer.
check out : -
www.moleskine.com