Basically, it's not very good.
Herbalist is quite a strong combo with it, always use the buys, get lots of them quickly by returning the potion, make them strong late by returning them.
It's good with familiar too - your deck is bloated, the game is long, your deck is weak, and you already have the potion.
But to make it worth it, you need the opportunity cost of the potion to be worth it, which means you need some way that makes the game really long and affords you the time to buy the potions and the stones.
In fact, it's not that great with most of the other alchemy cards, because they tend to work best when you're drawing a lot of stuff!
Um, what else? Sifters are good, but usually you don't have time for them.
I guess it's halfway decent against mountebank? But when do you have time to buy it in a mountebank game?
The ambassador issue... well, if you're doing this as a counter, you want to buy duchies a lot sooner than you otherwise would. Actually this is probably true in general for p-stone. But it's not a very good counter, just because, well, you don't get to play it very much. Usually, if it works as a counter, silvers would also have worked.