I created a circuit in Eagle to test the resolution and fills using a fine tip pen in the Cricut.
The pen itself is a 0.01 inch diameter permanent marker, shortened to fit the machine. The test image is made of three identical copies of a diagram. The diagram contains some sample SMD and through-hole pads, as well as traces in 0.01, 0.03, 0.05, and 0.07 inch widths.
The legend reads "10mil pen, 20mil pen" and "30 mil pen" to indicate the pen size as given to the Eagle CAM processor function, but the entire drawing is rendered using the 10 mil pen in order to see which circuit will have proper fills with no white lines inside.
The results prove to me that the best fills do indeed result when the pen size is accurately set in the Eagle CAM processor. When the pen in smaller than Eagle thinks, it doesn't lay down enough lines to completely fill the solid traces. The TQFP and SMD resistor pads look nearly usable, even the horizontal jitter is almost nonexistent near the middle of the page. (Maybe the duct tape pen mount absorbs vibration better than my custom metal pen holder?) The worst flaw just seems to be incomplete coverage in the diagonals of the largest (0.07 inch) traces, which should be easily retouchable by hand.
Here's the workflow:
1) Create the board in Eagle
2) Run the CAM processor, select output device HPGL, layers Vias, Pads, Top or Bottom, and Holes.
3) Start a new drawing in Inkscape. Import the HPGL file into Inkscape. Select the imported circuit diagram, and click "Copy".
4) Mount the pen in the Cricut tool holder.
5) Start Make-the-Cut, and do "Paste in Place" (ctrl-shift-V) to paste without resizing. Arrange as needed and then cut to the machine.