I'm not sure where to begin review such an amazing place. I'll start with the hosts because they drive the personality. Easy to deal with, amazingly responsive, helpful and they don't just rent you a room, they make you feel at home. I would gladly host them in my home without Airbnb!
The place is amazing. The location is fantastic and the view lives up to the billing. I expected the "view" of the Astronomical Clock would require leaning out the window. Not so! Every night could watch from our perch. The double windows create a quiet sanctuary even though you are in the heart of the square. The furnishings give it a great Prague charm and the kitchen is fully equipped (not that we used it). One note on location: the benefit is being in the heart of all the locations you want to visit so if you are looking for more local places to dine, you'll reverse commute out of the square. Martin gave great recommendations and walking to the edge of Old Town / New Town border where beer drops from .99 to .39 and the food quality doubles takes only 7 minutes.

I will stay here again if I'm back in Prague