We have used FP in the past but gave decided now its not for us but we have met lots of people who think it's brilliant.
We have had 3 successful visits but a few others that weren't. You are usually put on a small piece of land that can't be farmed and this may be quite rough or muddy. In fact one such farm we had driven to for some distance was so bad we turned around and found another place. Another farm where we actually bought some delicious cheese we were plagued with flies from the cattle. We also stayed on a vineyard with a lovely pitch where we bought some gorgeous Pineau, still trying to find it again.
If you use FP then you must be prepared to move on if it is full or you don't like the place you are put on. The other thing is although you are allowed to stay without buying their produce I think you would feel uncomfortable if you didn't.
I think it's worth trying at least for one year to find out if its for you.