My mother, my husband and I stayed 4 nights in Ellen's apartment. It's very centrally located, an easy walk from the main train station and close to the main attractions. The street numbers are a bit confusing though.
My mom did the check in work Ellen as my husband and I arrived later. It went smoothly and Ellen explained her everything. She also had a folder with tour suggestions, restaurant recommendations and useful things to know.
The apartment itself is beautiful and decorated with taste. Very clean too. There is A/C in both bedrooms and the living room, and it's very quiet. The apartment was very quiet as well. On evenings, the street is animated with two restaurants and a bar in the street, but it didn't bother us and it wasn't till too late.
The stairs were quite narrow and steep though, with no elevator, so it might be challenging to carry heavy suitcases upstairs. It wasn't a problem for us though.
Overall, it was a fantastic experience and I definitely recommend Ellen's apartment for a stay in Florence.