We chose this place for the central location and the lift....and the lift was broken! Judging by reviews, it had been broken since at least May. We were never notified about it, so if this is an important issue to you, I would ask Ds the status before booking. This place is on the 4th floor (5th in American terms) - not the most fun with suitcases or after Arthur's Seat!
The location is very good for exploring the city. Just keep in mind that although it is very close to Waverly, you'll either take a quite inclined road, or a very inclined stairway shortcut to reach the flat. There might be a better way that we didn't realize coming out of the station. This is another thing that we didn't know to ask beforehand and would have been good to hear from the host on the day of check-in.
The flat does its job. It's quiet in the bedrooms, which are on the side opposite to Royal Mile, but you can still look down to it from the living room to see all goings-on (a few parades and the Queen, in our case!) and listen to all the bagpipe music you can handle.
The kitchen was very nice and updated. The flat was clean, but in need of a bit of TLC and decor perhaps. It wasn't as nice as the photos made it appear. The beds were a bit thin and springy and the pillows weren't great. The shower was a bit weak (the flat is on the top floor).