Overall, the flat was lovely and its location was fantastic. It gives you easy access to both the Old and New Town and is within 5 minutes of the train station. The flat and especially bathrooms were very clean, with excellent pressure, hot water, etc. We found the check-in and check-out easy enough despite the extra walk.
However, there were a few unpleasant aspects to our stay. (1) The massive windows that look out down from the Mound (which are obviously a selling point in the photos) were very dirty. This definitely detracted from the living space. (2) The floors creak loudly everywhere throughout the flat so that anyone waking up early and walking around will inevitably wake up other guests. (3) Lastly, the television sound/speakers in the living room make it virtually inaudible - though of course you're not in Edinburgh to watch tv, so only an issue for a half hour or hour each evening before bed.