in my experience w/ several f*ck buddies it seems at some point after you have been buddies for a while one of you wants it to be more(in my experience it has been the girl) and that usually messes it up.
the best f*ck buddy i ever had was dating a guy who took her out to dinner and bought her flowers but didn't bang her right so she would come visit me
(maybe that isn't a f*ck buddy but it was great)