I would expect that people would count this as an Alchemy card and play by the 3-5 Alchemy card recommendation. Of course if it were the only potion card it wouldn't be good, but that's the case for any potion card.
I have to concur with others who have chimed in here. Although Potion is certainly skippable more often with only one Potion card on the table, I think Transmute and Possession are the only two you wouldn't often go for under those circumstances. Although the 3-5 rule is recommended by the Alchemy rules, it's just a recommendation, and the Potion cards were, according to Donald, purposely designed so that they might be desirable in the absence of any other Potion cards.
Philosopher's Stone - Anti-synergizes with most of the other Potion cards anyway.
University - Fewer Potion cards probably mean more targets for this.
Apothecary - Seamlessly integrates into a deck and makes that Potion hurt less besides. Some of the best kickers are the non-Potion cards Coppersmith, Vault, top-deck manipulators, and +Buy (there are no Potion-cost +Buys).
Alchemist - When Alchemist stacks work at all, they work just as well without assistance from other Potion cards. Best kicker is Herbalist, but most any source of +Buy will do.
Golem - With no Village, and especially with deck-junking, this can be a powerhouse.
Vineyard - Yeah, you'd like to have University, but this is good on any +Buy board, and Workshop/Vineyard is fun.
Scrying Pool - Heavy trashing and a few non-terminals (or better still, Villages plus strong terminals) make this great.
Familiar - Great players can find ways to skip this sometimes, but for us mere mortals, ignore this at your peril.