For a little while, I've been hanging out and playing Dominion (and thinking about trying other games) on BrettSpielWelt, a nice German online-boardgames site.
It is definitely a different, and inferior, experience to playing these sorts of games face to face.
Faster setup and teardown is wonderful, but is met with the problems of network issues, players mysteriously dropping, a pace that is too fast, lack of trash talk, and turning it into a pure mechanical and mathematical exercise.
But what I came here to say about it was:
I've never played the Dominion Alchemy expansion before; I've got it on order, to arrive along with my preorder of Prosperity.
I have heard that the game is "best" with 3 to 5 cards from Alchemy or none at all.
However, in my experience on BSW with "all random", at least purely mechanically, it doesn't seem too disruptive to have just 1 or 2 Alchemy cards in the mix.
When you normally play Dominion, there are typically a couple of cards in the Supply that just don't get bought anyway.. and this doesn't seem to grossly affect that. And it still gives you an option; sometimes just Golem in the supply is worth buying a few Potions, or whatever. It really depends on the environment.
(Also, I've had my good 4-cost cards Swindled into Potions countless times; that's really obnoxious.)
So, I think I will favor "all-random", or at least "not artificially affecting the number of Alchemy cards in the Supply", when I end up owning this expansion, and we'll see how it goes.
Adding both Alchemy and Prosperity to the card universe at the same time is going to lead to a lot sparser distribution of cards from any particular set anyway.
As for the BSW interface.. well, at least games are over in 20 minutes or less, as opposed to JOL or BSG PBF.. ;)