I had a positive experience at Casa Billi. The couple that runs the place are extremely friendly and knowledgeable about Florence and their neighbourhood. I found the place very quiet (I stayed in one of the rooms at the back) and clean, just a stone's throw away from the sights and the main train/coach station (Santa Maria Novella). Remember that Florence is extremely compact and one is never too far away from a David or a Duomo. Enjoy.