I have bought a bunch of the leveling stones. 3 Charms per stone. I like using them to get the last 2 levels of recently captured wild pets. I figure there is no point in saving them so I am burning them as I go.
I did buy one of the 15 charm stones to upgrade one of the spores I collected. I still need 1 more for the last spore I have that is not rare. Probably the same boat as Pyy, a bunch of the new pets turned out to be flying so I burned all the flying stones I had, one of which was the wild caught Shadow Sporebat
with so few spots to hunt them I just decided to stone the first one I captured.
If something comes along that is worth buying I'll start saving them, but for now burn baby burn.