The most amazing aspect of this place is the view! We sat for hours on the little veranda looking out over the fields, watching sunsets, deer, rain, hunters, and, in the evening, the distant lights of Siena coming on. Other pros - communication with Catherine and with the caretaker, Adi, was great. He was responsive when we asked about wifi issues and when we had a problem with a key breaking off in one of the doors. The town of Montisi is charming. There's a coffee place, a couple of restaurants, and a small market. The hours and days of the week that these places were open were hard to grasp. We were there in October, and one restaurant was closed for the season (Da Roberto) and the other one, Il Barrino - l'Ombelico del Mondo, was only open certain nights and had no sign. When we did go to this one, the evening was great. The walk to town is lovely through olive groves, but it's a good mile and a little rough going on a gravel road. And dark at night! Montisi is in an EXCELLENT location for visiting wonderful towns nearby.
The cons. The house is small, and there are many concrete steps - you need to watch your step carefully throughout the house. The outdoor area is wonderful, view-wise, but there are few places to have a comfortable sit or lie-down (and look at the view). The lighting throughout the house is dim. The kitchen was stocked with very basic equipment. There was little or none in the way of essential items like salt, sugar, dish soap, coffee pods, and paper goods. We went to the market in Sinalunga to buy groceries, a good well-stocked market, but we were surprised at our need to buy the basics. We like to cook and found that the equipment was spare (no roasting pan, mixing bowl, etc.)
A major issue was the internet. Even when it worked, it was very sluggish and prone to frequent random outages. The phone service is also spotty. So if connectivity is important to you, this will be an issue.
We did love the place with all its quirks. We got used to them and spent so much time swooning over the view that the other issues faded as the week went on.