We have stayed in Fattoria Bacio for one week in June/July 2010. Fattoria Bacio houses are 5-10 minutes ride from main (SP79) road (GPS 43.585645,11.088424). If you go from Tavarnele to Certaldo, turn right in Betto into dirt road.
We occupied Azalea stone house, wich consists of two bedrooms, one kitchen/living room and one additional small room. There is place for four/five people (or up to six people if you really must). It is not luxurious, but for sure comfortable and clean. All guests can use swimming pool. We have used it almost every day. Owner of the Fattoria Bacio is very pleasant and warm-hearted.
The best place for making grocery shopping was COOP in Tavernele, then COOP in Certaldo. You can also buy something for the breakfast in Fiano if you really must.
While spending vacation in Fattoria Bacio be sure to visit San Gimignano (***), Firenze (Florence ***), Siena (***), Piza (**), Chianti (**). Almost everything is in one hour driving range. Only Piza is 80 km. But... If in my opinion, though attractive you can make additional Chianti and Florence trip and see Piza on pictures.
If you like to make reservation please do it in advance. Some rooms/houses are reserved one year ahead.
I can easily recommend staying in Fattoria Bacio.