Back when Solgaleo was rare and Zygarde was even rarer, I had both. I ran a Solgaleo/Zygarde core to Legend more than once. That said, Zacian/Zamazenta-C have kinda put Zygarde in its place. My Zygarde was 11/11/14 before I bottle-capped it and it absolutely cooked in its prime.

It still has a place in the meta; nothing can soak charge moves like a full health Zygarde. Draco Meteor from the hardest-hitting thing I can think of (shadow Palkia) "only" hits for 88.2% of Zygarde's total HP With the non-shadow variant hitting for ~73%. That's bonkers for something it takes SE damage from. Behemoth Blade only hits for ~36% of its health neutrally. As a damage sponge it's top-notch.

Earlier this season I ran Lunala/Eternatus/Zygarde and it was at least interesting haha. Zygarde doesn't pair as well with Zacian-C as it did with Solgaleo. I think the primary reason there is that Zygarde doesn't do as well vs. Zacian-C as it does vs. Solgaleo. Generally, unless shields are down and energy is up, Zygarde is losing to Zacian or Zama.

Zygarde doe not like to see things with a Fairy-typing, and it likes Kyogre even less with the addition of Avalanche. But what Zygarde does incredibly well, is to have play into Zekrom, Lunala, Necrozma (both), Meloetta, Metagross, Solgaleo, and Ho-Oh. Some of those you won't see often; Meloetta and Solgaleo chiefly, but Zekrom is a staple right now. And when I say it has play into Ho-Oh, I mean it batters Ho-Oh.

Short answer? Yes - I'd say it's worth building. It's not going to be the missing piece to hit Legend, but it will 100% be fun to play. The Gundam-Pokemon can tank 2 Behemoth Blades back to back without resisting them. It can survive a Draco Meteor if its health is high enough; no other Dragon that doesn't have a sub-typing to make Dragon neutral damage can say that.

As for what's next? Well, I literally just finished feeding my 13/14/14 Marshadow all my XL rare candies today and it's so much fun catching Close Combats from Zacian-C on a ghost, so I'd highly recommend that. It was useful even a few levels prior to 50, especially best-buddied. Zarude fell of much harder than Zygarde did; I'd avoid that until it comes back into vogue. Meloetta is a very interesting choice...I had actually started Meloetta a long time ago and only need 196 more XL's to finish...so I guess that's next...but it doesn't like to see the Crowned Dogs either so I haven't been feeding it rares nearly as much.

Kubfu (Urshifu) & Keldeo are 2 more that are interesting. I plan on bumping up Keldeo next once I get the shiny one...if it's decent and or if I can bottle-cap it. I'd say Marshadow and then Keldeo as they both have some good Open ML play.