Six of us stayed here for two weeks, and it was great. Some specifics:
1- The location cannot be beat. The apartment is a five minute walk to the Duomo, Piazza della Republica, the Uffizi, and many of the other great sites in Florence. The apartment itself is on a slightly quieter street, and two of the three bedrooms are located off the street, so noise wasn't much of an issue. Also, we found a ton of great restaurants very close by.
2- The apartment is huge. There are three great-sized bedrooms and plenty of room for others to sleep if necessary. There are also four bathrooms, a nicely-appointed kitchen with a little sitting area for breakfast, and a legitimate dining room. The outside space was also very nice. Overall, the apartment was large, clean, beautifully furnished, and extremely comfortable (we used the AC on one or two of the hotter nights). The apartment really had everything we needed (including a stroller and crib for our son).
3- Isabella and her son Francesco were extremely responsive and helpful. We had a couple minor issues (the showers were draining slowly) and they were fixed within hours. Isabella was also kind enough to do some research for us on restaurants that could accommodate my son's food allergies.
This was my first time using airbnb, and the experience was perfect. I'd highly recommend staying at this apartment.