I have the HDZ and the kevlar belts from reprap.nl
My experience with the kevlar lines up with the excellent work in the belt stretch thread in that they seemed to roughly halve the positional error and backlash I was seeing with the original (non steel) belts which were about 9 months old.
OTOH, using the bitzero I am still consistently 0.1 to 0.2mm off from the side of the stock when setting zero, I have to put ± 0.1mm of backlash compensation in the cuts and this varies from the side to the middle of the machine so I am really not in any position to tell anyone how to kill their backlash or positional inaccuraccy issues 
Over to the more experienced folk