I went to Florence in October 2005 and my sister recommended this apartment. The owner usually rents it out to people who actually want to live there, but she also rents rooms out for extended stays (for instance, people on holiday.)
We stayed in the "Casa Carolina" apartment. It was a BEAUTIFUL apartment in a good location. It was walking distance to many famous places (Ponte Vecchio, Il Duomo, as well as outdoor markets and various restaurants). There is one master bedroom with a double-sized bed and a cot. And there is also a pull-out couch in the living room. It's a good size and fits 5 people comfortably but there were 6 of us and we did not kill each other!! (I think on the website it states it is possible to add a third bed on request.) It has a kitchen with utensils and cookware, toaster, microwave, etc. and also a washing machine, color tv and air-conditioning. It also has a little "relax" room and 1 bathroom. Upon arrival we were given complimentary grapes and a bottle of wine. :) The place was also very clean and reasonably priced. I cannot remember the exact rate we paid but we stayed there about 5 nights and I believe it was $550 ($600 at the most, I'm only guesstimating) for the 6 of us.
Furthermore, we had a perfect view of the Piazza del Mercato Centrale (where the apts are located) and there is an EXCELLENT restaurant called Za-Za's on the corner. (Apparently a lot of famous people eat there and now I know why). There's also a market across the piazza where you can buy food and utilize your kitchen if you like to cook. :) My friends and I did not venture into the market however b/c we only learned it was a market on our last day in Florence. Didn't really matter though b/c none of us cook.
Overall it is a beautiful Tuscan-style apartment that fits up to 5 people comfortably. It's clean, in a good location and reasonably priced (esp if in a larger group). If you want to experience living like a Florentine and not stay in a hotel, this is the place!!