I don't know if this is relavent but I actually work in a shoe store that sells these shoes and in my experience:
-No one with any problems with joints (hips, knee, ankle etc) should wear them. They can cause problems with stability and hence make you fall.
-They take a decent amount of time to get the feel for.
-they generally are only for walking
-they are very soft. which I think is where most of the appeal is.
-the advantages in toning that you get from the shoe would be equivalent to a 15 minute workout doing something along the line of pilates, focusing on the legs and core muscles.
Ultimately I would say that if you are only buying these shoes as a means to toning your body, I would advise investing in a cheap home work out DVD or putting the money towards a gym membership. If you are only looking for a comfortable, lazy, everyday shoe then go for it. Why do I say this? because most of the claims that they make about improving fitness can be achieved simply by walking for 30mins a day anyway.
Hope that helps and I'm not repeating anyone else