I'm also working on some improvements to reduce the amount of plastic used, decrease the footprint, and increase the drawing area. :)
The robot is theoretically capable of very fine control. A 1/2 degree rotation of the X or Y servo theoretically equates to about a 0.2mm movement in the corresponding direction. Since my pen tips are about a 0.5mm in diameter, this is pretty good.
However, the play/backlash from the existing plastic rack/pinion design is a major factor. This is mostly a function of a plastic rack/pinion rather than ground precision metal rods and expensive bearings. With improvements in the design including the exact way in which the rack/pinions mesh, I think I could improve this a great deal. I could further reduce the backlash by using herringbone rack/pinions, but I'm trying to refine the rest of the design before I come back to this.
The top post on the site, http://shor.tw/11d, has a couple examples of my recent drawings.