I still am far from convinced that a one time use coverage move makes up for giving up a useful item AND a moveslot. Yeah, Gems were great in past formats. Yeah, we know too little about this whole mechanic to really judge its viability, but personally, I don't see it.
It would be a shame. Every time Pokémon introduces a new mechanic and it fails it's specially sad. The essence of Pokémon are the Pokémon itself, and by catching those Pokémon what you get is variety. Variety of types, attacks, stats, natures, designs, recolors (yes, shinies), genetics, abilities... Everything in Pokémon mechanics, or in its core playability we should say, is about variety. It’s what keeps you entertained, catching them all and using them in different ways for battling purposes (or breeding, to get more variety again, which let you use those new Pokémon for battling; one thing leads to another). As I was saying, everytime Game Freak introduces a new mechanic without success, it's detrimental in so many levels it can't be compared with other games from the JRPG genre. Pokémon doesn't have (and to me, it doesn't need either) that story, plot, argument, narrative enfasis, whatever you call it, caracteristic from the JRPG traditional genre. It doesn't have that character development, intriguing plot, main adventure of 50 hours (look at the Tales of series --well, these are ARPG but anyway--, Final Fantasy, Persona...) and that is what, I think, we like about Pokémon. Its superficial simplicity and its concept, which differs from the JRPG standards.
Pokémon, more than any other game from the same genre, needs to refresh its air. It needs to constantly include new things (call it Pokémon, call it mechanics of any sort, like mega evolutions) because it can't afford to use something like a new story as a novelty. Generation VI did well, incredibly well with mega evolutions. Yes some of them are broken, yes the metagame is centralized about mega evolutions (is this bad to begin with? We have had 17 years without megas, it was time for a change in the rules), but they feel new, the feel successful, they changed your mind "oh, now I have to forget some preconceptions and learn". And to me as a player, from a playability perspective it's great. Generation VII includes new Pokémon, includes the Alola forms, it will include new items for sure, but the only mechanic, the only thing that is really new and it's going to add a new rule to the battle system are Z Moves. Of course the metagame will develop, some Pokémon will go up while others will go down, that happens everytime, but in the end we will be playing by the same rules. Of course a game is something more complex than a few mechanics, there's so much to evaluate, gen 5 is considered one of the best if not the best and it didn't include any deep change like the previous generations (holding items, natures, special-physical split, abilities...), and it was one of its biggest flaws. The games were better, different, more rich in content, but the mechanics were the same (at least compared to what i said before, or the actual mega evolutions).
So, I hope, I really hope Z Moves succeed.