We enjoyed the stay at Massimiliano's Borgo Flat. The beds were comfortable for two adults and two teenagers and the air conditioning worked well ( though understandably strained with 100 F heat). The apartment is quiet at night, and essentially all you need for seeing the sights!
We did not find the narrow steep stairs a problem, but we are fit and had small carry-on luggage only. Those with mobility issues or full size luggage might have problems. We were sad to not find wine glasses and a pasta strainer/sieve. These would seem to be standard equipment for an Italian apartment! However, it did not damper the enjoyment of our meals.
The location of the Borgo Flat is great for seeing the sights. It was only about a 10 minute walk to the river, 20 minutes to the Duomo, about 30 minutes to the Accademia. You want to be on this side of the river, as there are less tourists and better restaurants.
Max was very quick in his communication, and I recommend the Flat!