Yesterday, I ran 4 miles in my Merrell barefoot shoes. Calves are sore, but not horribly so. It took me over a year to get used to running in them and that was after I started wearing them everyday to work. I had fits and starts with them and running and had to keep going back to my Nike Free shoes. Got a nasty case of bronchitis (or the flu) which knocked me out of exercising for almost 3 months. Finally this spring, after alternating them with the Nikes I've been wearing them every time I go out. I do have to make sure I stretch my calves after a run or walk, and this helps a lot.