No personal experience, but a woman at work has had a pair of Shape-Ups for a few months now-- she reports no weight loss or increase in muscle tone, despite wearing them regularly. Small sample size, so YMMV.
I have heard some anecdotal reports of them being unstable (particularly on treadmills/ellipticals) due to the curved sole, resulting in a predisposition to injury-- any minor loss of balance can result in a fairly significant fall.
You're probably better off with a pair of cross-trainers.