I would get it last. Alchemy is a small set with some great cards and a few terrible cards, and considering how small the set is, you're looking at just not that many great cards for the price. Also some people just really don't like the Potion mechanic even though it is totally fine.
Best Cards ("fun" not power): Vineyard, Scrying Poop, Alchemist, Apothecary, Apprentice
Okay Cards: Golem, University, Familiar
Trash: Transmute, Herbalist, Possession, Philosophers' Stone
I'm sure some will disagree with me but Possession is just very difficult to play with in a real game of physical card Dominion. It gets confusing, you have to constantly hand cards around or move around the table, and the game ends up generally being kind of shit.
Overall it has some great and powerful cards but I wouldn't hesitate to get every other expansion first.