If you try on the Shape Ups or knock offs in the store, you will feel the muscles that will get a work out as you wear them.
The balance ball sandals (Tone Ups) aren't comfortable in the way that normal sandals are.
The instability they provide really kick your butt. I feel the muscles working like crazy to keep my feet balanced in each step.
I'm lazy, sort of. Things that make work outs easier yet provide the same effect are appealing to me.
I don't like the hype on other things, but the hype on these are true.