I was traveling with my 3 year old daughter and my parents. First of all, the apartment's location was WONDERFUL! It's down a small cobblestone street (there's a sidewalk, which was nice since we had a stroller and rolling luggage), and directly across from the churchyard where Adam Smith is buried.
We were also very close to Holyrood Palace and its parks and gardens, and got to climb up around the crags and my daughter was so proud on her birthday, to have "climbed a mountain!" The area was gorgeous and with amazing views, even if you couldn't climb up too high (my mother's knee isn't great, but she was able to come up a bit too). I walked to the Sainsbury grocery store (all the cashiers I had were SO nice) each day pretty much, and there was a Tesco across from it... all conveniently located if walking isn't a problem for you. Even with my mom's bad knee, we made our way to Holyrood one day, and up to Edinburgh Castle another day, with lots of shops and cafes to stop in and take a look on the way. (I'm sure buses would have made it all even easier, but we walked.) Oh! And the Museum of Childhood is also very close and toddler loved it.
The apartment itself is only one flight up. And it was clean! The space was great for us with 2 bedrooms with nice big beds (!) and lots of pillows (that I used as a barrier on my daughter's side of the bed, to make sure she didn't fall off... bed was so big that I still had plenty of room to escape her sleep-kicking). The living area was very nice, with a view of the churchyard beyond the wall. Gary had a lot of guidebooks available and a parking spot for us too, if we needed it. The kitchen had everything we needed and more--fridge, microwave, utensils, kitchen table where we did cake and candles for my daughter and father's birthdays, as well as things like sugar, spices, etc. We weren't able to figure out the clothes dryer part of the washer-dryer though there was a manual and maybe we were too tired. But there was a nice big clothes drying rack that was great.
We don't have radiators for heat where we live, and though I warned my daughter about not touching the radiators, she accidentally did and cried a bit. So if you have small kids who haven't been around radiators (or heated towel racks in bathrooms), that might be a thing to be aware of. Most of the doors in the apartment are kept open with doorstops, so it's important to make sure your little kiddos don't mess with the doorstops because it felt like they'd slam shut when released.
We really liked the place and would recommend it to others.