Anyone use Sprenger jointed stirrups?

I have heard they are good for riders with knee and ankle pain. I have also heard that they don't feel nearly as stable/secure as a regular stirrup. I am borrowing some to try, but I haven't been riding much this winter, and my horse and I are too out of shape to give them a real workout. If you use them (or tried and discarded them), please let me know what you think. Also are the off brand like this: a href="http://www.doversaddlery.com/horse-s-jointed-stirrup-irons/p/X1-0735/cn/4721/" rel="nofollow"http://www.doversaddlery.com/horse-s-joi…/a as good?