I wear a size 13 in Nike but 12 in most others.  I ordered a 12 and they were slightly small but since they're flexible and worn without socks they were usable.

The deal breaker for me, and Im surprised no one wrote about this in the reviews, is the fabric on the insole.  Theres a hard plastic thin mesh type material on the top of the insole.  Its incredibly uncomfortable to walk on.  I can feel in scraping all the bottom of my feet.  I cant imagine how much this would tear up my feet if I walked far when my feet were wet.  They super comfy with socks but unwearable barefoot.