I love the massage things on the soles of my feet.  they are awesome.
however, when I set the strap to fit my feet, the front and back edges dig into my feet, rubbing blisters and creating hot spots.  In order to keep this from happening, I had to loosen the strap so much that the sandals fall off my feet pretty easily.  they are okay for sitting in a chair, but not so great for walking, and if I had to move quickly, they would be left behind.