I and my girlfriend stayed in Alessandro's place, and we strongly recommend it. The house is very spacious and nice, and has very positive vibes, maybe as a consequence of Alessandro working with energetic therapies. We felt very welcomed. Alessandro took the time to explain the city to us, show a lot of maps and give tips, and the room itself is full of material about Florence available for the guests.
We actually chose this room because of convenience, location and price (especially because there was free parking space), without giving much attention to the idea of it being an ancient chapel with frescoes from the XIV century and all, but we were really impressed by how beautiful the room is, as well as spacious and well furnished. The frescoes are really there and, without being specialists, but having gone to a lot of museums during our trip, and also talked to some Italians about it, we ended up convinced that it's all for real -- apparently it's not that rare to have this kind of historical building in Italy, so it might end up on AirBnb for quite an affordable price.
Apart from being very beautiful, the room also has very nice lighting, a lot of space, and even its own full dinner table, with plates, wine glasses, olive oil available, etc., so you can (as we did one night) cook in the kitchen and go have a meal in your own room.
The only limitation of the house, mentioned by some reviews, is the fact of there being only one bathroom, but at least for me that wasn't a big inconvenience. There don't seem to be other rooms for rent in the house, so the bathroom was used only by us and by Alessandro, his wife and their daughter. The bathroom has a lot of space and they have a lot of towels so everyone is quite comfortable.
I strongly recommend staying at this place!