We stayed here for 5 nights when we were visiting Edinburgh. The location is awesome. It's right in the grassmarket, and you can see the Castle from both bedroom windows. There are a bunch of great restaurants right there in the square, and the flat is centrally located next to all the great sights. It couldn't be closer to the castle.
The flat itself was great for our family, and felt like home while we were there. It had everything we needed, including room for our two girls to have their own room, separate from ours, a living room, and a kitchen. We really enjoyed this place and would definitely stay there again if we were in Edinburgh!