I just beat the game so I'm getting ready to get back to Mega Evolution soon, but here's what I like about the Z-moves.
The Mega concept was a neat one but very limited considering less than 1% of all Pokemon get to use it. I'm seeing some very creative strategies built around the Z-Crystals so far that I'm appreciating because of the added diversity/options.
It's been mentioned on this and other websites that a lot of people are more excited about the stat boosting Z-moves as opposed to the "Mega Attacks", though in-game the giant cutscene moves have definitely helped me. I'm liking the Porygon-Z/Z-Conversion route
Smogon's already had at least one person talking about Z-Splash (+3 attack stat boosts) and I've kept it on Mimikyu to potentially power up Shadow Claw and Play Rough, but I haven't actually used it to see how much of a threat Mimikyu could be. This is what I currently have in my game and I want to test it out:
Mimikyu @ Normalium-Z
Splash
Leech Life
Shadow Claw
Play Rough
idk about the EVs and what not but I know it's got a higher Physical attack stat vs. Special.
I do have to say though that even after Disguise mine seems to take hits pretty well. Also Verlisify posted a video about Extreme Evoboost and Eevee potentially hitting like a truck afterward, so we'll have to see if the possibility of having a competitive Eevee is a thing.