It's not the lengthiest expansion, but it's still a pretty good 8 hours of content, more if you actually care about filling out the dex or getting all the digletts.
The new characters continue on SW/SH's trend of having some of the best characters - Avery/Klara are great rival additions that also show off the "minor/major" league system of gyms a bit more. Aside from that, this DLC has a vastly improved wild area - biome diversity, a sea to surf around, a few small caves, digletts to search for with rewards for finding them, and a few character cameos scattered about. Other positives are the 100+ new Pokemon available, more raid dens, some higher leveled content (although the level scaling isn't perfect - trainers don't evolve their Pokemon even if they're 60+ with scaling, and if you're too high of a level you'll just breeze through the content), along with a nice postgame episode featuring the best rival/professor Hop.