First of all, AMAZING location. I'm not sure there's a better place as a base of operations in Edinburgh - short walks to great cafes and restaurants, literally steps from the National Museum, minutes to Grassmarket. Check in was smooth and easy, and Jon went absolutely above and beyond for us when our flight arrived early in the day and we needed a place to drop our stuff (six hours before our slotted check-in time).
Apartment was clean, wifi worked well (once we figured out that we had to turn on the outlet the router was plugged into). The shower was a tiny bit tricky at first, but once we figured it out it was fine for the duration of our stay.
The ONLY complaint we had was regarding the heat. It was quite chilly in Edinburgh while we were in town, and we never did manage to get the radiators turned on. There are three in the apartment, and the only interaction possible is turning them on at the outlet (which we did). After three days, finally (the morning we left) the one in the living room started to warm up - the others were cold to the touch.
Jon did thoughtfully include a pretty powerful space heater, but as there were two of us in the bedroom and one on the futon in the living room, we couldn't really use it to warm all of us while we slept - our friend on the futon wound up sleeping with multiple layers and a wool hat the first night. Edinburgh warmed up a bit after that, so we didn't make an issue out of it - but we thought we should mention it regardless, in case future travellers book the apartment during colder weather.
If we were just doing something wrong, I'd love to know what it was - but when I asked about it after the first night, I was told "just turn on the outlet, that's it" and cautioned that it could take a while for the heat to build up. If that's the case, it must take longer than three days with the outlet on, since two of the three radiators we're still cold when we left.
Not a reason to avoid the booking at all - everything else was amazing, and we loved our time in Edinburgh!