I know most of the racers prefer DID erv2 520, but I'm wondering if there are other brands of 520, like the RK GB520GXW or even the DID VM 520 that hold up well. Any luck with Tsubaki or EK 520s?

I used the DID ERV with a VM clip

I will be running the RK chain you mentioned next season. They are all but identical to the DID ERV series to be honest and you save some great $$$$. It is a realy high end chain too, In fact I believe Mick Doohan was one of the guys who did testing for it and ended up racing in Gp on it a few years ago so it cant be too bad!
-Shane

i run the RK X ring chains

i run what ever the hell SHANE sent me ..... http://www.gixxer.com/forums/images/graemlins/laugh.gif

just ordered DID x ring