Overall a pleasant stay but cannot recommend presently for several reasons.
Pros--
Location is an easy walk to High Street/Royal Mile, shopping, restaurants, and Edinburgh Castle. A slightly longer walk to Holyrood but still quite do-able.
Comfortable mattresses
Excellent linens and towels
Apartment was clean
Bathroom was functional and clean, albeit somewhat cramped
Kitchen was well appointed with basic equipment, coffee, tea.
Main bedroom was fairly large, second bedroom smaller but still adequate.
WiFi worked well
Hosts were easy to contact by phone, and via AirBnB site.
Cons--
Noise. Loud drunken people stumbling by each evening woke us up from sleep--and I am a heavy sleeper. My daughter slept with earplugs after the first night. The rubbish collectors banging bins etc were not a pleasant way to wake up in the early morning either. I have lived in Manhattan, NY, and this was equivalent.
Odors. Musty smell when we walked in, which we later figured out from the nonfunctional washing machine (see below). Intermittently a septic-tank like odor in the bathroom, too, though everything was perfectly clean, no overflow/backup, etc.
Nonfunctional washing machine, Had planned on doing laundry--indeed it was one of the reasons we chose this flat--but it didn't work when we got there and was filled with fetid smelly water. Didn't notice the water until we had already tossed our clothes in, which subsequently smelled horrible and then had to be washed by hand try and get the odor out. Not pleasant. Given our short stay it was not able to be fixed by the time we had to leave. The water clearly had been there for some time.