I used to work on an arctic research ship and they gave us two pairs of these about 8 years ago.  One pair I've never even used.  I used them sometimes when I was working outside from 0 to -40 degrees.  The rest of me was freezing, but my feet were always fine.  Now I mostly use them as slippers around the house, they're thick enough to handle that.  Just a couple little holes (I think where they were originally attached together) and still going strong.