I started using a pen for drawing in my sketchbook because unlike pencil or charcoal, pen drawings don't smudge. And unlike watercolors, I don't need a complicated setup to start making a mark. Of course, the downside is that once you've laid down said mark, there is no going back, you've committed to it.
Despite that, I feel drawn to pen and ink sketches. Maybe it's from all those years of doodling in my notebook in school instead of taking notes!
Koi, originally uploaded by flyingroc.