First, Leonardo is an excellent host. His welcome and check-in were the best we've ever had with AirBnB. Using our limited Italian and Leonardo's decent English we were able to understand everything we needed to know about the apartment as well as helpful tips about the area (where to get groceries, close restaurants, etc.).
The apartment is great. Literally a stone's throw from Piazza Santa Croce and a short walk from most of what else you'd want to see in Florence. Location is really hard to beat. The size was perfect for two adults and a child in the second bedroom. You could do two couples pretty easily as well.
Leonardo goes to great lengths to make you feel at home and provides some necessary staples that other hosts have required us to purchase. This saved us a trip to the grocery store the day we arrived so we could just settle in and enjoy our first evening in Florence.
Our only complaint (truly the only complaint for an otherwise perfect stay) was the level of noise coming from the street. We stayed over a weekend during a period when school was in session (and maybe also due to students from abroad? we heard a lot of young English speakers) and the Friday/Saturday night noise went on until the early hours (2:30 a.m. and later) and the sound bounces up the buildings/alley to the bedroom windows. We were able to adjust after the first night with our own sound dampening measures, but the windows do not insulate much at all.
Having said that, we loved the apartment and Leonardo is a wonderful, gracious host and we would stay again. If you are a light sleeper, though, be prepared to address the street noise for the weekends. (It may be less of an issue in the autumn/winter and during the week, too.) If you're not a light sleeper, this place is a 10 out of 10.