Worth it? Fit Flop muscle toning shoes

These Fit Flop Pietra sandals are the newest addition to the line and I'm pleasantly surprised because they are relatively fashion forward (especially for something that's an orthopedic/athletic slipper).   My 8-months pregnant friend was wearing them today and has found them comfortable so far.

They're sold at Amazon or Bliss for $59.99.

 

Fit Flop sandals and boots are basically shoes that supposedly help you tone your legs while you're wearing the shoes.  It does this with a sole "composed of three distinct densities of EVA foam that together create a slightly destabilizing effect, your muscles are engaged each time you step, increasing leg, calf and gluteal muscle activity for better tone"

Their shoes are also supposed to decrease stress the amount of stress on your lower back and feet. I can't vouch for the former benefit, but can vouch for this aspect.  I have a recent nerve pain on my right foot which has made most of my shoes (including flats!) pretty uncomfortable.  But for some reason, I can walk for hours in my Fit Flop sandals.

Do any of you own Fit Flops and what are your thoughts?
Most importantly, are they worth it?