We were there for the Fringe Festival and we could not have asked for anything more. You are about as centrally located as possible. Close to the the Royal Mile, and right across the street from one of the main venues for the Fringe. EVERYTHING is walking distance.
The room was clean and comfortable, up a one-story walk up (nice wide staircase).
Facilities are great, a well-stocked kitchen, and a washer/dryer.
It did get loud on Thursday and Friday nights. There is a club downstairs and you will hear and feel a lot of it. Didn't really bother us because it fit our schedules. But be aware.
The flat is above the hosts' bar, and that is where you get your keys. A huge advantage is having the staff available to you for questions and help. They are super friendly and helpful. We especially appreciated that they were there to handle the delivery of our lost luggage -- we did not have to stick around to wait, wasting our festival time.
All in all, a great place, supported by great people. Loud, but when you're in the middle of a party, what do you expect?