We are using it since R2 already and it is working so far. However 15.1 has some problems when it comes to software upgrades.
I described it especially for EX2200 here: http://forums.juniper.net/t5/Ethernet-Switching/EX2200-system-overload-problems-with-15-1Rx-upgrades/td-p/302166
But also on EX8200 I had much trouble as NSSU is not working properly. NSSU worked better in 11.4/12.3 time.. I now avoid NSSU whereever I can and if I need it test it on non-production equipment.
Note that this info is from my experience in the pre-recommended phase of the 15.1 release train. I just upgraded my test systems to R5 coming from R4 and still had 8200 troubles. We will see how R5 has developed. Also in the past the R5 releases have often been the first which were to be thought of as "mature". I usually still avoid using pre R5 releases but this time had to use it due to some policy saying I may not use software that is not receiving updates/bugfixing like 12.3 (that was before Juniper decided to extend support by one more year...).