Sun and Moon are very give and take titles. Gen 7 brought new life into previously irrelevant Pokemon such as Torkoal and Pelipper, but also nerfed the likes of Gengar (which was utterly pointless when it utilising levitate wasn't as much of a concern since it went Mega) and Talonflame. All the Alola forms are great additions that help make some old Pokemon fairly relevant (Muk, in particular). That said, competitive play was probably at its best or at least second to Gen 6, though other online features took a major downturn with the Festival Plaza being slow and pointless compared to the Gen 6 GTS.
For the gameplay elements tied to the main story, perhaps the biggest improvement is HMs no longer being necessary, which offers a lot more options in customising a team instead of forcing you to have a Fly, Surf and (sometimes) Strength user regularly. Island Trials are also something of merit, adding a bit more than the standard formula of gyms and evil teams. SOS encounters are pretty annoying for finding rare Pokemon though, especially considering some Pokemon are found exclusively through SOS.
As for the story itself, it's one of the better entries, even if nowhere near something like Black/White. The truth about the Aether Foundation was pretty predictable, though at least showed a bit more effort than the standard and the final battle is pretty much fanservice done right in a callback to a certain glitch battle in Gen 1. The characters are, for the most part, a bit more developed than those in about half of the main series games, Guzma being a particular stand-out.
As for whether you should play it though, I'd say it depends as Ultra Sun/Ultra Moon offered a fair bit more in some areas. Whilst some elements of catharsis is lost in the finale, some aspects of the story were improved such as Lusamine's motives being a lot more fleshed out. USUM also has a lot more to do in the post-game such as more Ultra Beasts, Ultra Wormholes allowing for you to catch almost all past legendaries (if you have both versions, at least), the Rainbow Rocket is pretty fun as a series of battles despite the idea essentially being the equivalent of a bad fanfiction and several other elements. If wanting a challenge, Ultra Necrozma is a far harder boss than anything that Sun/Moon's story mode offered too (depending on your team. I barely beat it in my first playthrough, yet oneshotted it on my 2nd playthrough with Alolan Muk's Z-Move Crunch).