Fantastic location, and nice flat! 3rd floor flat in a tenement which is typical of the gorgeous buildings in Edinburgh. Close to loads of nice restaurants / cafes. I recommend Bentoya for Japanese, and Lovecrumbs for coffee and cake. Despite it's central location (you can see the castle from the front door), the area is nice and quiet.
The flat is nicely decorated, comfortable beds, strong hot shower, nice kitchen. For check-in you need to go to a place near Waverley station, so factor that in before booking any taxis / other arrangements. The fridge/freezer was broken during our stay, and the hosts have provided a small bar fridge as a temporary replacement. They kindly provided a cot for our 1 year old (who loved the spaciousness of the place). The kitchen was missing a few basic items (only 3 mugs provided, no corkscrew or tin opener), but I imagine that will be easily sorted. All in all we had a brilliant festival week, and will certainly check the availability again when we plan our next trip. Thanks!