Came here with my family to see Edinburgh, and also a bit of the Fringe in August after the Lockdown was lifted.
The flat is fantastic, it's really clean, spacious, and well decorated. The rooms were really comfortable, and we did spend a few mornings lounging around flat, which was really enjoyable in itself; I found after traipsing round Edinburgh we needed a few rest days.
In terms of location, really good, a short walk to the center, perfectly positioned for both Princess street and the Royal Mile. If you were feeling lazy you could just jump on at Haymarket station and go the one stop to Waverly. If you weren't up for that there is loads of stuff around the actual apartment itself.; my personal recommendation is Monty's (a whiskey bar).
The only slight downside is that it's up three flights of Stairs, but that's ok I think as you have to do do many hills in Edinburgh, just remember it's the worst its going to be the first time when you have to drag all your suitcases up with you.
The Hosts are really great and responsive as well, providing you with lots of information for your stay.
I would stay again A+++++