So, I actually stayed at Il Borro 10 years ago. I rented an apartment in the village for a few nights. While it is was a lovely trip, it is very isolated from Tuscany and town. You definitely need a car.
I also stayed at Terre di Nano, an agriturismo right outside Montepulciano.
Very relaxing but not too far from town. We took several days trips - the Abbey of Monte Oliveto; Deruta. Would highly recommend it.
http://www.terredinano.com/index.asp?lang=eng
Terre di Nano looks great. It's not a gated community like Il Borro that isolates you, and yet looks to be 1 five minute drive to Montepulciano and maybe a 15 minute drive to Pienza. It seems that this would be another great relaxing place to stay, and as a base for exploring the Tuscan countryside.