We stayed at this apartment for three nights in late July. As a base camp for a short visit to Florence, it is extremely well-located, roughly equidistant between the Duomo and Santa Croce, and walking distance to all the major tourist sites.
Although centrally-located on a busy (mostly pedestrian) street, the apartment is up several flights of stairs and is pretty quiet. The building has an elevator to assist with luggage, etc., which was a huge help as we were traveling with a baby and had lots of extraneous gear.
The beds are very comfortable and three of the four bedrooms are air-conditioned, so we slept well every night despite the heat wave (40+C / 100+F every day we were there).
The terrace is a really nice feature to the apartment. It is at the level of the surrounding rooftops and provides views of Santa Croce.
The only negatives were that the bathrooms were somewhat dated and not in the best state of repair, and the kitchen was not as well-equipped as at other properties we stayed at during our trip, so we only really used it for basic food prep.
Also, if you are arriving by car and have not driven in central Florence before, make sure to get turn-by-turn directions from Stefano and plan your route in advance - our GPS and Google Maps did not have accurate information about central Florence and it proved challenging to navigate to the apartment. Stefano offers a valet parking service that we planned to take advantage of, but on his recommendation we ultimately parked our car at a garage on Via dell' Oriuolo and walked our bags to the apartment. This was pretty convenient and the same price.