why would you prefer them over a good pair of racing flats ?
are you already used to barefoot running ?
I couldn't buy the FF up here so i got a pair of neoprene water shoes ( after reading about them on a barefoot running site) . They feel like socks with a soft sole. I also wear socks in them to fill the shoe more tightly.
I use them , increasing time gradually but i don't see myself ever going faster in them than with a good pair of shoes.
Now, maybe i'll be proven wrong in the long run but my first experience and feeling is that transition is slow. My calf muscles remind me how careful i have to be.