I get the general dislike of Transmute, but I'm pretty sure I buy it at least half the times it appears, maybe to be contrary, partly cause I feel smarter winning with it.
I still don't think I'd call it good, exactly, but it has plenty of uses. Ruins are a big one, even early you'd rarely prefer a ruin to a Duchy. Ofc it's better to just trash them for nothing rather thb a dead card early, but you can still have some exceptions like Battlefield. Trashing for a Duchy is the main thing I get out of them, nice endgame sometimes. Naturally much weaker with Colonies, but if the game is a bit slow it can be a huge help. Also this'll combo somewhat w/ things like Silk Road and Duke. Potentially Duchess. Groundskeeper to some extent. Also, multi-type cards can help. For instance, Mill to a Gold and a Duchy, and that pair is also probably better than putting your Transmute or another non-junk card on an Island. This could further combo w/ cards that gain from the trash.
Buying Transmute just fer Curses is not worth it usually, since that Potion becomes a bit of a dead card too. But if it's the only trasher this might make sense, particularly if Market Square is on the board for instance.
Buying it early for Estates is bad, I don't think I've tried that, but I'm not inclined to. Quite slow to get 1 Gold, just buy a Silver over that Potion if nothing else. But trashing Victory cards can still be really good, for instance if you'll be getting Estates(Wild Hunt, Ambassador, Triumph, Followers, etc.) Also if you get hit by a Swindler for an early VP card.
The main time for it is w/ +buy and another potion card, I definitely get it more times there. Plus if the Curses run out Familiars become pretty useless, odds are you wouldn't want one then. I suppose on a similar note w/ +buy and stuff you could get a bunch of Transmutes(also by Transmuting treasure) since your opponent might not since it has a bad reputation. That could matter if it's the Obelisk, or possibly for Orchard or even Tower.
I can't think of anything too noteworthy I've done with Transmute, but I generally don't keep track. Might post a game where I use it and they don't for posterity sometime. Still I think it's undervalued.