Sakura Pigma Micron Pens are the tits. The 005 is VERY fine. The tips are rounded, so the angle at which the pen is held and the pressure exerted will change the thickness of the line, somewhat. My girlfriend sketches with them frequently and can get seemingly impossibly thin lines using the 005 and a very light stroke.
I usually use an 01 for scribbling text and marking grids for the checks on my Goffs. Filling is done with either an 03, 05, or 08 - whichever I find first. Don't bother with Superfine Sharpies - they have the reddish tint to the ink some people have mentioned, while Micron's don't.
The ink in Microns also has a bit of flow to it, which is helpful. It won't run at all under normal use, but running a fine tip over a groove in
SM power armor a few times will build up enough ink to flow into the bottom of the groove, blacklining the groove completely. The ink also stays tacky for a little while over paint, as it isn't absorbed as it would be by paper, which means that with quick work and a bit of tactical smudging by finger or paper towel, instant weathering is achievable on checkered designs.
I have noticed that the ink sometimes stops flowing when attempting to draw on smooth surfaces, such as untreated plasticard or smoothly painted models. A few strokes on a rough or absorbent surface (paper, cutting mat, cardboard, etc.) will get the ink flowing again.
As an example, here's a (regrettably crappy) pic of a pair of
WIP orks. The checks on the shoulder pad and gob were achieved with a Micron pen over an Astronomicon Gray basecoat, with the aforementioned smudge-weathering.