Unlike Ellie I favour a Bic fine point and for many years this was my preferred medium. When I was a child one of the best things you could get to draw on was a shirt card, (the stiff packaging that used to be in new shirts), but it was too shiny to take pencil satisfactorily, so biros, readily available but frowned upon for writing, were the perfect answer. The medium is not as unsympathetic as it might first seem. With changes in pressure, crosshatching and the like it is possible to achieve a terrific range of tones including a very rich black.
At college I was a bit shy of using colour, so I carried on using biro, together with water-soluble fibre tip pens, on heavy watercolour paper. And these were large pieces, imperial or A1, so I spent a long time honing the technique. From college I began working for magazines. The biro work really suited the dark editorial subjects that I tended to be given. At this time, still working on watercolour paper, I didn’t do roughs so would often make holes in the paper by repeated erasure and redrawing; a technique that went out of the window with the introduction of laser scanners, which didn’t know what to make of holes.
I now work in variety of mediums, whatever suits, and I really enjoy colour. There are though still times when a bit of biro is just the job.