I wanted to give a test drive to the different printable lm8uu designs available on thingiverse, so I compiled 14 of them in a single plate.
Note that some of those might be a bit fragile when printed with a 0.5mm nozzle, for lack of infill between shells.
I only included stl files which were ready to print in a lm8uu format. Some other designs exists that need repair or customization to be used as a lm8u, I didn't select them here, you can find some in my bearings collection http://www.thingiverse.com/DeuxVis/collections/bearings/
I haven't done the testing / selection part yet, I'll also include some bearings milled from wood in the process.
I plan to try different materials too. PLA is the usual recommendation for printable bearings, but polystyrene, PET, polycarbonate could be good too as they are very rigid.
I think I read somewhere about using nylon - as it is auto-lubricating - but I have my doubts as it may be very sticky on the rod (being soft).
1) Print a plate. Probably best to use layer height 0.25 or less.
2) Use a 8mm rod to test fit and play of each design.
3) Select the best fitting and print enough to replace your printer bearings.
4) Run your printer for a while and tell us about the usability and long-term wear.