Mariangela's place is in a fantastic location, particularly if you're traveling to and/or from Florence by train. The walk from Santa Maria Novella train station is less than five minutes -- and it's a beautiful walk by the Santa Maria Novella church and the Piazza of the same name. Walking from the apartment to many of the highlights of Florence (particularly the Duomo) is easy and quick. None of the major sites are more than a 15-minute walk away, and most are closer than that.
The apartment itself is also nice. The living room window looks out right onto Piazza Santa Maria Novella (you do have to step up to see the view, though). It has a washer and dryer (the dryer being a rare luxury in apartments like these in Europe). It is not especially quiet, however. You can definitely hear people out on the Piazza at night. And if you're using both of the actual bedrooms, note that they are essentially separated by just a curtain, so noise travels easily between them. With four of us staying together, we occasionally had issues with running out of hot water for showers, but it was OK as long as we either had a couple of people shower the night before, or spaced out our morning showers by 30 minutes or so.
But these were all minor issues we were happy to trade for the experience of having such a spacious, clean apartment in such a good location in Florence. Mariangela herself was also great to deal with, always responding to messages quickly. We only stayed a few days, so we didn't have any serious issues we needed help with, but I got the sense that she would have taken care of any such problems immediately if they had arisen.
All in all, we had a great stay, and we would certainly return.