The location is great and Casa di Antonella is definitely worth the money.
Our stay at the Fattoria was just great. Both ladies (mother and daughter) were very nice throughout the whole time. The downside is the distance to Siena. We were In the Fattoria by bike and 10km up the Siena hill is not too much fun. We used the kitchen to make pasta, we had bought the ingredients at the nearby Coop. On one afternoon we enjoyed a little free time by the small pool behind the house. The breakfast was very nice too.
The host was very nice and welcoming when we arrived with our bikes and luggage. The room was quite small but sufficient, especially since you will mainly want to walk around the interesting city anyways.
Simple, clean, friendly. Good breakfast.