I'm primarily an old school B-50 guy. Yes, it does stretch some at the beginning but it settles in for me. I think that it's quieter on my hunting selfbows than the FF material. Also, I'm basically cheap. I've never been enough of a speed freak to make the switch. For me, B-50 is plenty quick enough in my hunting range of 0-20 yards. I suspect it might make a difference out farther with deer ducking the string, but then again, I don't shoot that far at deer. I rarely install tip overlays on my selfbows. If I decided to make a FF string for one of my selfbows, I would feel the need to. I do however, like Cody's idea of a FF tillering string. I think I'll make one just for that purpose.
That being said I have both materials, but my plentiful string box contains mostly B-50. I save the FF string material for my glass bows, mainly because that's what people want. I've noticed that my glass bows have a sharper, noisier "twang" with FF material than with B-50.