Procession and King's Court are amazing with Feast in the presence of lots of desirable $5 cards, especially if the $4 options are otherwise uninspiring and Silver isn't a good thing - so any board that favours a moneyless deck is good. Graverobber is pretty nice. Duke is also good, or any other reason you might want Duchies (such as near end-game).
The other case where Feast is pretty cool is on a board where there are a couple of competing strategies and it's in your interests to wait for your opponent to pick one and then go for the counter. Or similarly, in a 3P game where you're better off going for the strategy that the other players don't pick.
Also, Feast helps to counter Embargo. If there are Embargo tokens on Silver, or on the key engine piece, just buy Feasts and convert them. You may lose tempo against buying the card directly, but if you can't handle the Curses it can be worth the trade-off.