My wife and I stayed at Rosewood Castiglion del Bosco for a week during Thanksgiving, and it was one of the best travel experiences we've ever had.
The resort itself is beautiful, occupying the entirety of a small village at the top of a hill. The buildings have been incredibly well-preserved, and the rooms strike the perfect mix of rustic and luxurious, with modern amenities and comforts. (They are also very big.) The setting is remote - it takes about 15 minutes of driving down gravel roads just to get off the property - but that is part of CdB's charm. And in all events, there is plenty to do on the property; we never once felt isolated or bored.
The food and beverage program at CdB is outstanding and among the best in the world. There are two restaurants on the property, one more elevated (Campo del Drago) and one more casual (Osteria la Canonica). That said, "casual" is somewhat of an unfair description; though it is more casual than Campo del Drago, which has two (very well-deserved) Michelin stars and incredible multi-course tasting menus, Osteria la Canonica serves some of the best pasta we have ever had, with locally-sourced, seasonal ingredients. There is also a wonderful lobby bar, with a massive selection of liquors and cocktails, both classic and unique to CdB. (There is even a list of cocktails that showcase Tuscan ingredients and fruits and vegetables from CdB's own garden; these were extremely inventive and absolutely worth sampling.)
Eating and drinking are clearly important activities at CdB. In addition to the restaurants and bars, there is also a winery on-site. We had a great tour (thanks Francesco!) and got to sample some of the best Brunellos we've ever had. CdB also offers cooking classes (pizzas, pastries, pastas, etc.). We did a couple of these, and they were truly one of the highlights of our trip. Our teacher, Erasmo, gave us some great recipes and tips, but more importantly, created a really fun experience.
If you're not into cooking or wine (or just need a way to burn off all those calories!), there are many other things to do on and off the property - a bike tour of the surrounding area, tours of nearby towns and cities (we did Siena, which was spectacular), truffle hunting (thanks to our guides, Giovanni and Alessandro, and our "hunter," Brio!), swimming (in a pool with one of the most beautiful views in all of Italy), hiking, and more. The nice thing about CdB is is has something for everyone. If you want to spend your time relaxing, eating decadent food and drinking wine and living that "far niente" lifestyle, you can do that. If you want to spend your time outside, hiking, biking, walking, playing bocce, etc., or fill your day with activities, you can do that too.
All that said, perhaps the most impressive aspect of CdB is its wonderful staff. I don't know whether it's an Italian thing, a Tuscan thing, a Rosewood thing, or a CdB thing, but the staff at CdB was unfailingly kind and helpful. Special thanks to Erasmo, who taught our cooking class, the guys at the lobby bar (Geremia, Tommaso, Vito) and the lovely folks at Osteria la Canonica and Campo del Drago (Chiara, Mikal). While I mention a few names specifically, every member of staff was incredible, and they are perhaps the biggest reason why we intend to return to this wonderful place soon.