i use ballpoint pen because graphite smudges on my sketchbooks- i finish one side of the paper and flip it over to draw on the next page and the pressure transfers graphite and ruins my previous drawings. really you only have a few drawing implement options after that. i’ve used microns but i find the lines too thick and invariable. ballpoint pens are dirt cheap and you can get a nice gradient of line thickness/darkness similar to what you can get with a pencil but without the transferring. actually a ton of artists use ballpoint pen, like http://brianjarrell.tumblr.com/. its just really convenient.