I have a pair, which I would probably not have purchased if I'd remembered the ads. They are all right. I do find them really comfortable, but have definitely not noticed a miraculously toned butt or anything.
I bought an off-brand pair, but mostly I feel like they make me look ridiculous and they don't do much besides improve your posture. :/
If you flex your butt or tighten your ab muscles while walking, it greatly improves the impact of your workout, though!