My girlfriend and I stayed in Florence for 3 nights. We loved Marella's place. It's bigger than we thought it would be and is clean and comfortable. It also has A/C which is always a nice touch. Marella leaves a suggestion book which we used for dinner one night and it was delicious!
The location is amazing! The flat is on a side street just off of Piazza Santa Croce. It's quiet, but you feel like you have a place on the square. It's minutes away from some busier streets and the river. Also about a 10 minute walk to Duomo and Palazzo/Ponte Vecchio. My girlfriend and I would recommend this place to anyone staying in Florence!