Anna & Simon's place is exactly as advertised. We never got to meet them but communication worked out just fine.
The location is amazing - just a half block to Grassmarket and a quick walk up the hill to the castle. Also, there is an adorable cat cafe (and other good cafes) nearby.
The living room/kitchen/dining area was a bit bigger than than the pictures make it out to be. Tip: bring detergent if you want to use the washer/dryer. We loved all of the teas and snacks left for us.
Just note that the water pressure for the bath is low so don't expect a strong shower!
We would definitely recommend this place to friends.