Walter is the most welcoming host imagineable. He's very accommodating about arranging arrival, understands that your phone might not work in Italy, etc, and is a fount of information and hospitality. He went above and beyond for me in arranging for a very early morning taxi pickup, and made a second phone call to confirm it. (And the taxi company he uses is super-reliable; they were there exactly at the scheduled time.)
The apartment is well-equipped, clean and comfortable. It is rather dark, being on the first floor in a very narrow street (so there's no direct sunlight). The bedroom is even darker, with no windows except on some sort of airshaft. Good for sleeping though. I didn't spend much time there during the day, in any case.
The toilet is up several narrow steps from the kitchen -- there is a railing, but the access is quite tight, and if you have mobility issues, this could be a problem. The shower room is also tiny, but functional.
Location is great -- easy to find, easy to walk to anywhere from, and there is also a handy bus line that stops in Santo Spiritu, just one block away. And it's quiet, even though only a short distance from a very lively square.
Overall, it was a good choice for a comfortable place to stay in Florence.