I seem to be in the minority but I didn't find these comfortable at all.  They were way too narrow for my feet.

I bought 2 sizes to see which would fit me best.  I'm a size 7.5 with thick meriono socks.  The first one I tried was a medium and they were extremely tight, both length and width-wise. I wore socks with them (I always wear socks) and the back kept slipping off as I walked around the house.

So then I tried a large.  They fit me better length-wise and I no longer had the problem of them slipping off the back, but they were still incredibly narrow.  I wore them for a few hours around the house thinking they needed some breaking in, but honestly, I just could not bear them any longer. They just squeezed my feet too much.