My boyfriend and I stayed at Badia a Coltibuono for 2 nights and absolutely LOVED it! The grounds are beautiful and setting picturesque. It was amazing to think we were staying in an 800 year old building, and that we got to sleep in the monks' rooms! So at the same time, while the setting was beautiful, the rooms were fairly rudimentary and smallish, although we didn't feel like they were missing anything. The amenities in the bathroom smelled amazing (made with olive oil), so I very much enjoyed that bit as well! The place is quite isolated so you literally didn't hear anything at night, which made for a relaxing deep sleep. The air was cool and crisp when we went, so it was nice to sleep with the windows open. The breakfast was delicious and plentiful, and I agree with one of the other reviewers about the fact they could also serve it out in the gardens to make it perfect (instead of the breakfast room, which was still very pretty). Nevertheless the setup of the breakfast made it easy to get to know the other travelers there, and share stories and tips from their travels. Make sure to try the restaurant on site, which had delicious and unique Tuscan dishes, as well as book a wine tasting tour, which also gives you a history of the place. We did ours at 5pm, then got ready for dinner on site.
Last but not least, the service at Badia a Coltibuono was amazing, and they went out of their way to assist us when we needed it.
One tip I would recommend if you are planning on staying there -use a GPS! It's not the easiest place to find, although as you get closer you start to see the signs for it.