Hi, my name is Nela and I am a sketchbook addict :) It started when I began to draw during my electrical engineering classes. I got tired of drawing on math paper so I always carried a folder with a few A4 papers. But at some point in 2006. I bought a large notebook with clear sheets (for geometry class) to keep my ideas in one place (until then they were scattered everywhere), and that's how it started. This notebook wasn't very good, the paper was too thin for anything serious, but it's full of quick sketches and ideas. Next I bought a small spiral-bound sketchbook pictured below. Most of my "lectures are boring" sketches originated in it.
After that I bought a fancy Winsor & Newton spiral-bound A5 watercolor sketchbook that to this date has 3 drawings in it. Because it's fancy n' shit.
I got a teNeues notebook to replace the small one and I currently use this one the most since I have it with me all the time.
Next I bought two Lega-lega notebooks, one regular A5 and a small one with red pages. I considered the one with black pages, but I figured the red one will be more usable. I haven't used it much though as I rarely carry it. The regular one I use pretty often, I take it whenever I have room in my bag. You can see the scribbles I made on it here.
I had more that I ditched because they weren't as good, so I won't address them here. You may have noticed that I don't use a Moleskine. (EDIT: now I do.)
Pictured above you can see a fragment of unfinished Tarsatica, contemplating on the In obscuro logo and some jewelry doodles in the red notebook, a spread of the used up small notebook (posted: random typography, Lectures are back), a glimpse of the teNeues spread (posted: Summer sketches), and in the Lega-lega you can see a quick sketch of Animus and another that I won't out yet, it's a personal project I'm currently working on.
For now I've been mostly using ballpoint pen, mechanical pencil and fineliners, but I've started to use some color and watercolor pencils as well.
And as a bonus, here's a sketch from last night - Michelangelo's Bacchus.
So to wrap it up, get a sketchbook, or if you're a writer, a regular notebook :) You never know when inspiration may strike, so you better be ready!
EDIT: This post is a bit outdated by now, please hop on to My Sketchbooks Comparison to see some of my new sketchbooks and my opinions on them!