Just an FYI but 12.3 release goes End of Support, June 30, 2017. So you have until then, dpending upon your exact circumstances, to decide on any upgrade action. Juniper wanted to provide customers with a big enough window in order to make this decision. 15.1 will be the LAST release for legacy/older-EX platforms EX4200/3300/2200/45xx/EX8200.
Does not really answer your question, but I am planning to wait for another few months (3-4) before making a decision. I hope by then lots more info on 15.1 usage.
Just FYI.