I just go a sketchbook myself, I fill them sometimes but most of the time I don't carry them with me and my sketches and doodles end up in napkins and papers... I started one this week precisely :) Even if painting digitally I still need to paint the doodles and sketches on paper, it's for me faster to translate the ideas if I trust my hand to the random lines on paper... :) Even if then I start a "polishing" up process! :)