I really liked them at first... but, the more I play, the more I realize they may not be a good thing for the game.
They are kind of the opposite of Mega Evolutions for me, where Z-moves are fun during the creation of your team and really bring out the creative aspect of team-building, but are also unexpected bullshit during battles most of the time. It's really frustrating.
Megas centralized the game and limited your options, but I didn't mind them aside from that (well, aside from the broken ones). These are really frustrating, but it's fun building around them. What do (You) think?