I've been keeping a sketchbook since senior year of high school, when my AP Portfolio teacher had us make our own (before that, I just drew all over EVERYTHING). Since then, I've kept a devoted sketchbook...with the exception of the year after I graduated college; I needed a break from drawing after 4 solid years of art school.
During college, I went a little nuts, filling sketchbooks (like the one in the top right corner) every 3 months or so. I know there's a huge jump in sketchbooks between '02 and '10; I filled 13 more little guys before I switched to those larger, floppy Moleskins. I figured you might start to worry about my sanity if I posted 13 more pictures of me holding tiny sketchbooks...
The bigger Moleskins were originally meant for work-related drawing (as opposed to recreational drawing), I loved the paper and size, so they became the standard. That is, until now. I'm going back to the smaller, thicker-paged Moleskin. It holds ink a lot better (nib inking on the bigger ones bleeds like crazy), and it's a bit easier for traveling. Here's the first page of the new dude:

































