If you go for Farmland like crazy, you can turn your Farmland into a Province every time you buy Farmland, essentially turning all but a couple of your Farmlands into $6 Provinces. I've never actually tried this and it probably doesn't work very well, but it's an option.
Farmland can work well with a strategy that takes advantage of green cards, particularly Silk Road and Crossroads. But usually, I've found that Farmland doesn't work super well in any particular strategy. A move I like to make with it late-game when I have $9 is buy it and then turn a Gold into a Province, getting you 2 VP more than just buying a Province. That's the best use I've ever made of it.
Similarly, if you go for Gold like crazy, you can just buy Provinces.
Buying a Farmland and trashing a Farmland for a Province only nets 6 VP. Any time you do that, you could have just bought a Province if you had any other source of $2 instead of a Farmland. Further, other sources of $2 aren't nearly as bad to have in your hand as Farmland when you only have $5.
Ways I use them:
- As BubbleBoy mentioned, late in the game, trashing a Gold to a Province for 8VP instead of 6VP. Especially good in Gold-heavy games.
- Trashing a Curse into a $2 card, especially a late-game Estate. Buying a Duchy gives you 3VP, +1 dead card. Trashing a Curse to, say, a Lighthouse is 3VP, +0 dead cards. Trashing a Curse to an Estate is 4VP, +1 dead card.
- Trashing a card that's no longer useful into a card that's more useful. This is generalizing what LastFootnote said. Potion -> Gold seems to come up most frequently. Things like Witch -> Expand are also decent once the curse pile is empty.
- Estate -> Silk Road. It's marginally better than just buying a Silk Road.
- Copper -> Crossroads.
- Bishop fodder.
I've never used them as an integral part of a strategy, though. Almost always, they're just ever so slightly better than what I would have bought or a slight boost to VP totals in the final turns. I'd love seeing game logs and descriptions of situations where Farmland really comes into its own.