Alessandro's air B&B is a gem. Its located in the middle of all of Florence's big sights - so getting anywhere from your home base in a breeze, and takes no time at all. The apartment itself is great too. It has a nice open feel to it, and it's a pretty large, comfy space - especially for a European city center.
And Alessandro himself is just a great host. Easy to get in touch with, helpful if you need something, and very accommodating (he helped us check in a bit early, so we wouldn't have to kill time waiting in a cafe with out bags). If his place is available during your stay: book it.