S.M.
I love mine. I wore the MBT's for almost 5 years before they finally wore out. They cost me 300 dollars though. I didn't want to spend that again. I bought my shape-ups about a year ago and they are holding up nicely for a 3rd of the price. I use mine because I have bad feet and these keep me from putting much weight on my heels.
I'm afraid I couldn't disagree with Nikki more. I can stand much longer in these than other shoes. It allows me to rock back and forth instead of just standing around. Also, my calves are very tight. But don't confuse the difference between strong muscles and a skinny body. I still have fat on top of my muscles LOL!