Many, many years ago – back in the 80s, I purchased a set of Rotring Calligraphy Pens, a Rotring Sketching Pen, and the Rotring Art Pencil.
Although I rarely use fountain pens for calligraphy or sketching, I like to keep a few around as they are easily portable (as opposed to carrying around a bottle of ink) and are convenient for doing quick layouts and impromptu sketches.
However, the best purchase I ever made was the Rotring Art Pencil. Essentially a holder for 2mm leads, the Art Pencil is the finest, well-balanced sketching tool of any lead holder. Unfortunately, they have not been in production for many years and are difficult, if not impossible, to find.
I not only used mine for pencil sketching, but found it well-suited for silver point drawing. A 2mm silver wire sanded to a drawing point fit neatly into the holder and gripped the wire securely.
I have come across a few websites where artists have lamented the passing of this fine drawing instrument, and although the calligraphy pens and sketch pens still appear to be in production, I will miss the Rotring Art Pencil.