I was a little cautious about buying these since it makes sense that it would be a little unstable.  I have been wearing them for a few months now and I don't notice that.  These shoes are great, and I think my legs are also getting stronger at the same time.  If anything I think it makes you walk straighter.  If feels like it works the backside, from calves to butt.  I would definitely recommend this product and when it wears out, I would probably buy another one.