The flat is very well-located. It is about five minutes walk from the Vatican, a 15 minute walk from the restaurants and bars of Trastevere, and a similar walk from Centro Storico. You can stand in one of the balconies and see the Castel Sant'Angelo. (In fact, we would not have gone to this castle had it not been so close. And it is a rewarding place to visit.)
The flat has high ceilings, benefits from natural sunlight and is airy.
The host was kind enough to allow us to check in early. He gave us a walk through of the flat (what to do, and what not to do). He had also very kindly sent us a map of restaurant recommendations which was very useful.
We had a great stay in this flat.