We were in two couples and stayed at Francesca's lovely house, and had a wonderful time there. She received us very well, with a cake, and offered to help us with tips and arrange a car to take us to the airport. The house is even prettier than it appear on the pictures. She has a wonderful collection of books and the furniture and decor of the living room is very beautiful and elegant. The location is amazing if you want to enjoy Trastevere, and very close to the historic center.
However, there are a few observations which we must point out, that were not clear when we signed the contract: the stairs shown on the pictures at the entrance are only one of three flights of stairs, so take that in consideration if you have a lot of bags, like we did. Another drawback we had was that one of the bedrooms had two single beds instead of a large one, and the bottom of one of them was loose so every time someone laid down on it the mattress would "sink". The hot water of the showers did not last long; after three minutes or so the water became cold - especially on one of them (the one on the corridor) - so if someone else had to take a shower afterwards they had to wait for almost half an hour.
All in all, we had a good experience in Torre de' Mercanti, and would recommend to other people interested in staying at such a historical location - although there were those few quirks which we mentioned above, they were definitely not sufficient to spoil our trip.