This is my 1st Pair of FitFlops and I have had them for about 2 weeks.  They are extremely comfortable and have great arch support, I would want to wear them regardless of the fitness claims. They also do seem to work, nothing mind blowing, but different muscles are getting worked in these.  My calves were burning the first week and a half, but I do walk at least 3 miles a day from work to subway, etc. and I wore the FitFlops the entire time.

The FitFlops did jack up my toes a bit, the thong part that goes between the 1st and 2nd toe is quite hard and rubbed my skin until it bled on both feet.  ALL thong style Flip Flops have this effect on me so I rarely wear them.  I have built up some thicker skin, though, and now all is comfy.