A good FF string will do all the things LBR said, in spades. One tradeoff, generally more applicable to a recurve than with a longbow. FF isn't as quiet as dacron, so you have to do a little more work with silencing a 'curve. I think the benefits far outweigh that one disadvantage though. Don't put FF on a bow unless the bowyer says the bow will handle it. If it's FF capable though, then I definitely say use it.

By the way, a bit of FYI for the lurkers... Just because today's Bears will handle FF, older ones will not. I don't think they started using reinforced tips until about Y2K. Not sure about that, but I am positive none before 1990 were FF compatible, so check with Bear if you've got one made between 1990-2000. Definitely don't use FF on any vintage Bear recurve.