I broke my 4 piece straight away for a new 2 piece, but I have a special reason. Had heroic tier in all slots besides helmet and gloves. My gloves were LFR and I had a different heroic helmet. Just upgrading my gloves to a T15 normal mode piece was a big upgrade by itself.
As for the set bonus, it's amazing for both Beast Mastery and Survival. But it seems to make a much bigger impact for Beast Mastery.