I bought a Primary drive X-ring chain at the begining of the race season and it has held up great!! I put a pretty good pounding on my bike and i ride alot of miles and i have yet to have a problem.
They are unbeatable for the price.
I have not had a good experience whit the RM chains. First I tried the O-ring version. The chain broke after six months. They credited me the price to a X-ring chain. Less than a year later this chain broke. I have never had a chain brek in over 100k miles of motorcycle riding. I never buy another RM brand as it sure did not save me any cash. I am now getting into the second year on the RK I replaced it with. My previous chain lasted about 5 years.
I can see getting on defective unit, but two in a two makes me say "no more."
Ive been using the Prim Drv chain, the o-ring model with the side plates cut out to let dirt and mud out. My sons CRF450 has had it on for 2+ years, it is worn out now, but never gave a problem. I have one on my 525 now and it just works fine, no trouble at all.