I have experimented a lot with different pens on the eggbot and want to share some of my experience.
After a lot of trial and error I sat down and really studied the "geometry" of the eggbot setup and how to get the best results. I now have a basic understanding on how to set the pen arm height, pen-arm "Y-position", pen height in pen arm etc. When plotting the repeatability is now very good and I can plot the same figure twice with pen following the exact same track as it did the first time around. I Use this method for getting higher "saturation (deeper color) when using metallic paint ball point-pens. To get good repeatability i modified the original "presision coupler" as it was always slipping (even with very high spring pressure on the egg) the remedy was to replace the "abrasive paper" with a rubber washer carefully centered on the coupler, The rubber washer I used was a "standard" US garden hose gasket.
When it comes to creating the thinnest possible lines I have come to the conclusion that when using fibre-tip pens, no matter what tip size, the ink will "bleed" into the egg due to the porosity of the egg shell and create a course line.
I have obtained good results from using ball point tipped pens, in the plot below I used a Uni-ball Micro with 0.5mm tip.
To get proper pen pressure for this pen (the ball needs to "roll" on the surface of the egg to get the ink to flow) I have added a rubber band around the pen arm to "assist" the force from the stiff hinge.
As my artistic skills are very limited I "borrow" and convert images I find on the net as in the example below.
The original image in the example was "stolen" from a fantastic artist - Stoshi - see her blog page here: http://shoshiplatypus.blogspot.com
( I wish I had a fraction of the artistic skill displayed in her blog!! )
She called the image "La Bella", a great work of art! I hand-traced the image in Inkscape to get a SVG file.
Original image: http://i496.photobucket.com/albums/rr328/shoshiplatypus/Zentangle/46LaBella.png
My eggbot version: https://www.dropbox.com/s/9osu5lpvmvquxpq/IndianFace.JPG
The "hand-drawn" look is due to the pen tip not moving smootly over the "rough" egg-shell. I am currently experimenting with different methods to "smooth" the egg-surface. Will report here when I get good results.