A couple of notes:
1. Some cards like Mandarin do work with Wishing well, you just need to play them twice. That still is a crappy tradeoff (13 coins of value in 3 cards to get the effect of a 12 coins worth in two cards). **Thanks Lfn for catching the error.
2. Mystic is good when you are gaining cards and have no draw deck/discard. Say you chapel down and play an Iw, a Mystic can draw the card you just gained and play it this turn. Any time you have an empty drawdeck or discard, Mystic is an assured hit. This can also work with mandatory discards into emptiness - e.g. play Horse Traders, discard 2 golds, play 2 Mystics, and draw 2 golds. Conversely, Mystic isn't too hot if you drawdeck/discard empty and can't put cards in there, potentially it can becomes just an overpriced duchess!
3. Mystic also works very well with ultra-lean decks. Take a golden deck of Bishop/Mystic/Silver/Gold/Province/Province - you can always guess the sixth card (in other ultra-lean decks you just have high odds), when you, rarely, don't draw the Mystic, you can just bish a Province, buy a gold (or duchy), and then bish the gold. Again, you need to be careful that you can actually make use of the Mystic's potential draw, otherwise, you are just overpaying for silver.