Jenslabs has published a thoughtful and thorough evaluation of a number of currently available rollerball and gel pens. He tested them using his Circlon machine.

One thing that anyone who as ever built a drawing machine realizes, is that to get quality results you need a quality pen. There are millions of pens out there, but after a little trial and error I realized that rollerball pens or pens with gel ink are the best pen types for my machine. Both rollerball and gel ink pens use a water based ink that is less viscous then the oil based ink used in ballpoint pens. The Circlon machine sometimes move very fast, so the pen has to be able to release enough ink to make solid lines even at high speed.

This is an excellent resource for folks with other drawing machines, such as Egg-Bots and WaterColorBots. We’ve linked to it from our page about choosing pens for the Egg-Bot as well.