So, Soothsayer.
The unconditional Gold gain is just really nice, a lot of the time. You don't have to reveal a Province, or buy a Victory card, or jump through any of the other hoops that Gold-gainers impose on you.
Giving your opponent an extra card is really good for them, but okay, you also give them a Curse. If they can deal with Curses really easily--Hermit springs to mind--this is not a good tradeoff, and you are probably better off without Soothsayer.
The big question, I think, is Soothsayer vs. its competitors, Mountebank and Witch. I would you think you want Mountebank/Witch first, and then a Soothsayer on your next $5 (if this is a ripe set for Cursing). The reason is, you'll deal some of your Curses with your primary Junker, and then use the Soothsayer primary for the Gold gaining and some additional Cursing--you won't give them the extra card as much. That's what feels right to me, at least.
A word of caution: You might not want to play Soothsayer in the end-game! If there are Curses left, but like 1 VP is not going to make a difference, and you aren't going to see that Gold, and you're racing for the last Province, you are severely hurting yourself if you play it.