If there is such a thing as a pen geek, I am it. I first came across Rotring isograph pens when in architecture school, where we used them for technical drawing. These pens come in a variety of nib sizes, from the minuscule 0.1mm all the way up to 1mm. These different thicknesses can be used to create a line hierarchy – the more structural lines are thicker, and the detailed lines are thinner. The precision and smoothness of the lines are unparalleled.
Nowadays I mainly use these pens for logo design or for illustration. You have to be careful not the smudge your work, but that just means being a little more patient.