I personally found Royal to be very worth it for the refined battle mechanics, (gun ammo replenishing after every battle and allowing Baton Pass effects to stack add much more strategy to fights, not to mention Persona traits and Showtime attacks) general QoL changes like additional SP recovery items in Palaces and marking each confidant on the map and showing when they're available to rank up, making each trip to Mementos actually feel important by allowing you to apply stamps you earn while exploring to increase EXP, money, and item drops (and changing that damn song every now and then), and of course; the brand new semester which added a couple of my new favorite P5 characters and seriously reworked another to give him much more depth and presense in the story.
...However, If you're already familiar with the brand new story content and the twists and turns accompanying it, that's a bit of a knock against a full recommendation of it unless you're absolutely craving more P5 (like I was lol), but it's definitely still worth a look if it's on sale.