My first pocket notebooks were green AF-issue Memoranda jobs, that flipped open like a reporter's notebook (Federal Supply Service #7530-00-243-9366 - no kidding). I still have a few blank ones, but have no desire to use them.
At some point, I went with hardcover Moleskine pocket notebooks for a while, then made the leap over to the Moleskine Cahiers (smaller notebooks, with soft covers). It took me a while to get used to the smaller cahiers (and I don't like the word "cahier" - it doesn't exactly trip off the tongue), but eventually I was won over.
Then I heard about FieldNotes brand notebooks on Dan Pink's blog and have been using them for a couple years now. They aren't hugely different than the Moleskine Cahiers, but I like the story & vibe of FieldNotes. Plus, it's a better word.
Despite my history of changing brands, I have to admit I feel a little bit funny about a recent purchase. At the Newseum in DC, I picked up a 3-pack of WritersBlok notebooks. A set of 3 (80 pages each) cost $6. In contrast, a pack of 3 FieldNotes (48 pages each) costs $9,95 and a 3-pack of cahiers from Moleskine, 64 pages each, go for about $7.
I haven't used my new WritersBlok notebooks yet - still have plenty of pages left in my current FieldNotes. But it'll be interesting to see what it feels like to try a new brand.