For some reason, slippers are the only footwear that make my feet feel claustrophobic; I have no idea why.  I absolutely love these slippers!  There's a tiny lip of a heel around the back of these slippers, but not enough to cup around the back of your entire heel -- just how I like it.  They are amazingly soft and warm.  I am a shoe size 7.5-8 and bought the L (8-9).  Get these, you'll wonder how you ever lived without them.