We (two adults and two children) were in Athens for 10 days in winter and had a wonderful time at Kosmas' house. The location is perfect. There are many very nice restaurants around the corner, and especially the fact that you can walk to the Acropolis is a real highlight.
The house itself offers more than enough space and the heating in winter worked great.
Kosmas himself is very nice and always answered immediately. We can highly recommend the house and would book it again.