Fairgrounds is a goofy card because if you can get it to 6 VP each, you're looking at at least 48 points plus likely a province, Duchy and Estate thrown in for the unique cards (58 points; 60 if you don't trash two initial estates). But if you can't - and your opponent can end the game before you get 15 unique cards, you're taking a 16 point hit, and you're at 42. This is seven Provinces, or six provinces plus Duchy and three initial Estates. One more Duchy for your opponent and you the Fairgrounds player lose. This is the calculus you need to think long and hard about when you think about going for your second or third Province (if you get four, you've likely won anyway). My thoughts are that you should get that second Province if it will not affect your ability to hit 6 VP/Fairgrounds or you can end the game on Provinces.
As for your opponent, him hitting a Fairgrounds is much like a non-Gardens player taking a Gardens. It's a 3-4 point loss for him and a 2-3 point gain for you, a five to seven point swing. For your opponent, that Fairgrounds is likely to be worth 4 VP to him but at a 6 VP point loss to you - a 10 point swing.
I think HME is correct that a good Fairgrounds deck is engine-based that can consistently hit $12 with at least two buys. This will get you double Fairgrounds, and then allow you to pick up miscellaneous parts as you close out and empty piles. Focus on the engine parts first, then move to Fairgrounds (they'll likely be worth 32 VP total at this point, so you've got at least 35 points available to you barring Estate trashing and a Province/Duchy buy) and final unique cards. Then end the game on piles - Fairgrounds and two engine piles, curses (if present on board, making the game more of a slog than it already is), or Estates.
My other thought was that you can try to end the game by getting half plus one of the VP available. This is 48 + 48 + 24 + 8 in a Fairgrounds game, so you need 68 VP (assuming no Estate trashing) to get there. 8 Fairgrounds + Province/Duchy/3x Estate and then two Duchies and an Estate get you there.