I've never used them in this application but on other things my experience has been disappointing. I found they had more friction than ball based linear bearings and coupled with a belt drive that may lead to juddering or stepping, which really wouldn't be good on a 3D printer. Might wear faster as well which will cause play (again not good). Though they may be quieter.
Depends on the manufacturer. Surely IGUS and other quality brands are much better than the cheap Chinese knockoff. IGUS is excellent!
I have only heard of one other printer that uses them so far (a kit). Nylon is self lubricating and some have printed their own linear bearings so they can't be that bad . If I can source them I might just try them out, I'm just crazy like that.
I bought the printrbot kit and those rods are hardened. I've also built a cnc milling machine and I'm familiar with the 'silversteel' problem you're describing but this is about the bearings and not the rod.