We had a great stay in Edinburgh and at Yvonne's apartment, which is located in a very calm and nice area.
It's also at a walking distance from the main attractions in the city (Old and New Town, Hollyrood Palace, etc), however, it's definitely not right in the center, so keep in mind that it can take up to 30-40 min walking to get to these places.
The aparment looks exactly as in the pictures, it's clean and it has all the necessary services (but make sure you bring your own plug adapters if you need them for your devices, as these are not provided).
Our first impression when we got to the building was not that great, as the entrance is a bit old and looks worse than the apartment itself. This was not a big problem for us, though.
Finally, we loved that Yvonne had prepared some information about the city and the apartment's surroundings (restaurants, pubs, etc).
This is definitely a good place to stay if you're planning on exploring the city for a few days!