Over the last year, I switched to Rhodia pocket notebooks (I think that they're the #11). I finally ran out of my beloved green Memoranda books and needed something new. They're great--easy to write on, nice paper, reasonable price. The one think that I don't like is the relatively flimsy cover. If you keep it in your briefcase, that isn't an issue, but mine gets bounced around in bags, jammed in my pocket and generally exposed to the elements. If Rhodia made a more durable cover for their small notebooks, they would be ideal.
Turns out that they do. I was in a stationery store in Tokyo a couple of weeks ago and saw them in the Rhodia display. I picked up one and have been using it with my pocket notebook ever since. It's great. A basic web search shows them selling for $10 to $15. Since you can replace the insert with a new notebook as necessary, I think that's a pretty good investment.
(Photo from Writer's Bloc)