Like many old school mappers I do like my pens. I'm not overly fussy about them though, and at the £1-£2 each price range I'll happily try anything that comes along.
For the last year or so I've mostly been using UniPins, but recently I picked up a full set of Microns and decided to see how they compare in a crude test.
Here's a 600dpi scan of some text (more to show which pen is which) and some boxes. All the pens have been used for a while, but the Microns slightly less than the UniPins. The Staedtler Triplus is included as a sort of control as I think it is a good all round choice that is not only cheap and widely available, but also comes in 30 different colours.
My initial thoughts are that the UniPins give smoother lines,but the Microns produce better black levels.