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.