I was excited to find this product in a style I thought would work for a variety of situations. Many people have complemented me on these sandals, not knowing they are fitflops.
The first week I was so sore I had to take a few days off from wearing the fitflops. I jog 3-4 miles a few times a week, but do very little weight training.  I'm in decent shape, but was no match for the fitflops at first. At first they feel so comfortable you don't think they'll do much.  However, those first few days I became as sore as if I had done lounges or squats for the first time in my life.  It was exciting and challenging.
They won't replace cardio, but are great for toning.  I'm very happy with my purchase and will probably buy another pair sometime soon.  They have really helped define my leg muscles and lift my butt.  Also, I used to have a fair amount of swelling in my feet from day to day activity and the fitflops have significantly reduced swelling in my feet. It might be the arch support.
I was skeptical, but am now a big fan of fitflops.