Ok, I have never been in the situation myself... but there was this girl in my year who was. At first I think she enjoyed it, for obvious reasons, but after a few weeks/a month she became really sad as she felt she was being used and it finally clicked that a relationship wouldn't form. She had to learn to get over him, despite not actually having anything more than physically with him. I don't believe they are friends anymore.
I believe that some girls think that FWB will eventually lead to something, but more often than not, it doesn't tend to. The girls are the ones that tend to suffer the most at the end of it all.