It's pretty good. I liked it. It's a decent fantasy epic adventure with a likeable cast. However its story can get a bit abstract and nebulous when it comes to the seraphim, the purification of malevolence and a few other examples of made-up airy-fairiness. I happily watched the whole series, but then afterwards I happily deleted it and I can't see myself ever wanting to rewatch it.
It's based on a Tales game. These go back to 1995 and there have been previous manga/anime adaptations, but every Tales game is a standalone with its own cast, story and world. They don't cross over, unless a game gets a prequel, a sequel or a spin-off. (As it happens, Tales of Zestiria has a prequel game, Tales of Berseria, which is set thousands of years earlier and gets a partial adaptation in eps.5-6 of this anime.) They're all high fantasy, though.
The world of Zestiria includes:
(a) a fictional continent called Glenwood that's divided between two hostile countries: the Hyland Kingdom and the Rolance Empire.
(b) fairy people called the Seraphim who are invisible to all humans except...
(c) "Shepherds", i.e. humans who can see Seraphim. I think you only get one of these alive at a time, if you're lucky. A Shepherd can gain magical powers and will be regarded almost as a messiah.
(d) "Malevolence", a magical evil force that builds up from mankind's negative emotions. It looks like really bad weather if you're magically sensitive and it can turn beings into monsters called "Hellion". There are also dragons.
The problem with this show is that it exists on two levels. Let's call them "Fairy Level" and "Human Level". The first is a rather abstract level in which our hero, Sorei, becomes a Shepherd and hangs out with lots of Seraphim. This leads him to wander around playing Jesus and purifying malevolence. (This is theoretically a lot like fighting monsters, but in a non-violent way that doesn't hurt your enemy and doesn't really make much sense to the audience. You accept it, of course. It's fine. It's what the story is. However it's always a bit abstract and I don't think I ever really cared.)
Most of Sorei's friends are Seraphim, which means they can't be seen by ordinary people and they can't even touch anything. I think. I'm not sure about that last one. This means they're permanent bystanders who just follow Sorei around and can't get any story of their own. They're nice, but they don't matter. They can also jump inside Sorei for fights as a power boost or something, which I'd call an anime being driven by game mechanics rather than audience considerations.
All that I enjoyed. It's fine. However it doesn't have great weight and you're basically watching nice people do abstract fantasy stuff. "Human Level", though, is actually worth watching. There's Aliisha (who's great) and Roze (who's pretty good). Aliisha is a princess, a knight and a tough-minded idealist, which unfortunately puts her at odds with certain people at the Hyland court who rather fancy the idea of going to war. Aliisha's the reason to watch this anime, basically. I like Sorei, but Aliisha's the one who does real stuff like conveying emergency supplies to disaster victims, confronting warlords, having to fight her own friends, etc. Aliisha vs. assassins is awesome. Apparently there are game fans who dislike this anime adaptation for sidelining Sorei and the Seraphim in favour of Aliisha and Roze, but personally I thought Aliisha's was the real story and everything else was the stuff that happened in between her scenes.
Roze's good too, though. She's got a dark side that Aliisha doesn't, although she's also lots of fun and you'd be happy to have both as allies.
The character designs and costumes can occasionally be distracting. Presumably they're straight from the game, which appears to mean more revealing costumes than you might expect from queens, princesses and government advisers.
I liked it all, really, but I'm less committed to the stuff with the Seraphim. (Unfortunately this includes the anime's last few episodes and finale, making them merely okay rather than actually good.) I liked the real world. I liked the dirty wars and the politics. Hyland and Rolance are both afflicted with scheming evil liars in powerful positions. I liked the bit in ep.16 where Sorei goes nobly bulldozing with his purer-than-pure principles into a situation where they might get a friend killed. After all, the stated theme of that episode is "justice within sight" vs. "unattainable ideals". However that's the good stuff. Probably the majority of the show is just Nice Sorei and Nice Seraphim being pleasant and watchable but not usually that dramatic. I'm mildly amused by two girls having boys' names (Simon, Ian). I like the Arthurian references.
Feel free to give the show a try, but the episodes between the good stuff (aka. Aliisha and Roze) can be a bit meandering. There's still some decent material with Sorei and the Seraphim too, but it might be more slow-paced than you'd expect.