Two of us stayed one night in mid March. It was wonderful. The service, the room, the food, the baths, the spa services were all excellent, and because we stayed on a Sunday night during the off-season, the price was very reasonable. Plus I cashed in a Hotels.com free room and it was a bargain. They even upgraded is to a room with a view over the pools to the mountains.
The weather was dreary with spotty light to moderate rain which seemed to abate when we were in the pools. Thank you ancient Tuscan rain gods. As a result of the season, the day, and the weather, we almost had the place to ourselves.
We stayed at the Adler several years ago and it did not measure up to Fonteverde. The view at the Adler is magical, but the restaurant, the service, the ambience, and the facilities are inferior. Plus it's more expensive. I checked it first.