I'm well into a multi-year campaign. Doesn't makes sense to introduce most of the changes at this point. I would rather join as a player in play-test one shots.
But I have thought about introducing the new exhaustion, inspiration, and crit/fail mechanics. These could be added pretty easily. But I don't want to have to re-jigger PCs with the new class rules when they are at 17th level and have been playing the characters for several years.