I have the ebay version on my machine. The step loss is usually 0-2 after a a bunch of milling.
You can tune the drives via the RS232 adapter cable and their utility.
By default it has an 1000 lines encoder. The driver can actually handle more (4000?), so you could replace the encoder and get (hopefully) more accuracy out of it. I haven't tried that yet.
Edit:
I also haven't done any tuning (508 driver model)