It's a gimmick. Shoes aren't going to miraculously give you a great bottom. However, if you enjoy wearing them, and they encourage you to exercise, you might just wind up with a great bottom as a side effect of wearing them... you'll get the same effect no matter what shoes you have on, if you exercise regularly
My coworker who is also a fitness instructor says they are just hype and don't really work, in fact she said one thing she guarantees you'll do is trip and sprain your ankle on them... My mom uses them but she says because at work they provide extra cushion for her feet.
My chiropractor said there is not a single study so far that proves what they claim. His recommendation was to buy some if you really like the feel of them and think they make you walk more.
I tried them several times and just found them odd, so I didn't buy any.
Well it may be the placebo effect but I always feel more energetic in them and somehow thinner although I haven't lost any weight. Even so they are amazingly comfortable. I think they help with posture as well which is supposed to make you appear thinner.
I have a pair (not Shape Up but Avi Motion) and I haven't noticed any difference with them. I wear them more for comfort and they force me to think about my posture than for any weight loss benefits. Granted, I am not going to snub my nose at them for that effect if it work. xD