I actually own a pair of Skechers Shape-Ups. They cost me $89, which is the most I've ever paid for a pair of shoes. I knew right from the start that it was a dumb idea, but I found the super-cute Mary-Jane style ones that would be easy to step in and out of for karate classes, were very comfortable, and well, I knew better, but what if there really was some weird fitness voodoo involved that would actually allow the shoes to deliver on their promises of a better backside?
Many, many women I know have been seduced by these promises, and some of them have been inspired by the shoes to walk more, which gets them results! Good for them, sincerely. I won't rain on anyone's parade if they're convinced the shoes are the key. But if anyone's still on the fence, tempted by the ads of every shoe company that's jumped on the toning bandwagon in the last year, here is one article from the American Council on Exercise, which investigates new exercise gimmicks and reports on their efficacy and safety. There are others like it; feel free to Google.
The basic problem is that the body adapts to slight challenges like those posed by the toning shoes very quickly. The shoes are supposed to slightly increase the motion of each step and create a balance challenge that the leg muscles need to adjust to, and adjust they do! I was a tiny bit sore the first time I walked with the shoes, and never again. My body had learned to walk with this new challenge, the muscles quickly adapted, and that was that. No further change occurs because the body, in general, wants to stay the way it is. Changing takes resources and the body is an efficient machine. In order to create change in the body, it must be continually challenged. That's why slow, prolonged exercise produces very little in terms of weight-loss. While it may make us healthier overall and decrease our risk for heart disease and diabetes, significant weight loss will not occur with this type of slow, repetitive exercise unless food intake changes drastically as well. We now know that techniques such as interval training and concepts such as muscle confusion are much more effective in producing significant weight loss than strapping ourselves onto a treadmill and walking for hours at a time.
So anyway, consider spending your money on a good cross-training or running shoe and mixing up your workouts (perhaps a session or two with a personal trainer?). Toning shoes aren't magic, unfortunately, and unless you want to spend a ton on what is otherwise a cheap most-purpose tennis shoe, you're probably better off passing.