Today, I carried one around and made notes and sketches, which was actually quite handy.
I got a bunch of exploratory sketches done and I wrote out the main characters' profiles.
This is a helpful system for me since I have 3 different note and sketch books, as well as index cards, where I've got scattered notes and thoughts for different stories. You throw in my digital notes, and I'm pretty much a mess. At least this way, I can scribble and sketch till my heart's content, and keep all focused on one story, in one notebook.