I bought two pair of these a year ago ... one brown, the other black.  The two pair were visually different and the soles were nothing alike.  I have worn them to work every day, rotating brown and black on a daily basis.  (Yes, I know!) Over the course of the year, the black pair has been replaced twice.  The sole comes loose after a few months.  I tossed the last ones in the garbage last week.  The entire heel covering had come loose and it sounded like I was wearing flip-flops!  The entire sole is covered with a thin layer of rubber, that's what comes loose.
The brown pair?  The are still doing fine.  The sole is not loose anywhere.
All I could say is, if you're looking at these in black, go look at something else.  The black ones are just not up to the Dr.'s quality standards.