What I enjoyed was the "Geocaching" aspects - going outside with friends, helping each other search together, visiting new places I normally wouldn't go to (ie new Pokéstops, catching a train to new suburbs to explore what's there). I also like the fitness aspect, ie tracking how far I've walked. But Pokéwalks with friends aren't fun when someone is more interested in defending a specific gym, rather than Pokémon being an excuse to hang out and go for a walk by the lake together.
I've spent more on this than any mobile game (probably $15 - $25, I kept needing to upgrade my Pokémon storage and bag storage). But I uninstalled the app earlier this week.