In my 3 playthroughs of this game so far, I've never bothered with the Fox S-Link.
Anything interesting happen with it? Do the foxes services become affordable eventually?
Eh, relatively. fake edit: Just wiki'd it, and it goes down from 60 yen per SP to about 15, so not too terrible. But with how easy it is to get invigorate this time around, I didn't find myself needing it too much. Fox Quest-line (it's not even really a story) is pretty meh, imo. Make sure to pet him when he's in a good mood for extra outfits inside the tv, though.
Invigorate + SOS + Arcana Cup + Tiny Soul Tomatoes means you need the fox much less than in P4 unless you're dungeon grinding at end-game and/or don't have any of sp items.
All in all, taking this much slower than I did P4, not trying to min/max s-links first time around, but I am trying to get max stats and see most of the new stuff. Having some of the S-links behind such huge walls (Tower, I'm looking at you) is still pretty lame.
Am in Mid-August, managed to one-day 4th dungeon with some crazy aracane selection + rare hand crits resulting in me being a fair bit overlevelled. Feels good. I think I'm about to get to some of the Golden exclusive stuff, which is pretty cool.
On a sidenote, one time, I saw Yosuke and (slight story spoilers, I suppose)
team up for a double attack-type thing, is that just a super-rare chance, or is there a way to make that proc?