(Mild spoilers for US/UM ahead.)
If you bought Sun or Moon and enjoyed it, and you aren’t the biggest fan of Pokémon, you probably won’t care to come back for seconds. Ultra Sun and Ultra Moon make some good changes, but they make them right near the end of the game, and for the majority of your story experience, you’ll be playing the same game. Of course, this won’t bother you at all if you missed out on SM, and if you did, you should get US/UM, because these are the definitive versions of the games. They made some changes that I didn’t particularly like, such as making there be an unlimited number of each Ultra Beast to catch, but most of the changes, especially related to the Aether Foundation and Ultra Recon Squad, are good and enhance the game.
The Team Rainbow Rocket postgame is genius. I love fighting legendaries from past generations, and the battle theme remixes are awesome. Hopefully the USUM soundtrack gets relased.
In closing, Pokémon Ultra Sun and Ultra Moon are nice sidegames. They provide a good value for the full game they’re bundled with, Rotom Visual Novel 2017 (original title: お前はもう死んでいる), in which you get to pretend to have affection for Rotom so he’ll let you gamble.