We stayed at the Villa Rioddi for one night on our passing trip to Volterra.
The hotel is within a easy 5-10min drive to central Volterra (apparently about 30 min walk, but I wouldn't recommend it and especially at night). There is free parking at the hotel. The hotel owners are friendly, laid back, hard working and can't do enough to help you. They gave us recommendations of places to visit, eat and park around Volterra. The swimming pool is a nice open area, with plenty of loungers surrounded by greenery and scenery. The breakfast as other reviewers have said is plentiful, fresh and majority home made.
Now the room - I would have given Villa Rioddi a five star review had we maybe stayed in a different room in the hotel. The room we had be given itself was clean, comfortable, had A/C, TV and all the usual things you'd expect from a hotel room - the only thing we found was that it felt like we was staying in am underground bunker with no windows. I'm sure the upstairs/other rooms would be a lot nicer but this was our only criticism of the hotel (and that other reviewers said that it is within walking distance of Volterra and it really isn't)
I would stay at this hotel again and would recommend it to others.