I cannot speak highly enough of B&B Agnese. I stayed at the location on Via Merello, which is a beautiful, tree-lined street within walking distance to pretty much everything, but far enough to be quiet at night. If you don't like walking, there's also a convenient bus. On the same street there's an excellent cafe/restaurant, Dispensa, where you have vouchers for breakfast (unless you opt to have the hosts bring specific foods to the B&B). Dispensa is also very good for lunch and dinner.
The room was spacious, comfortable and impeccably clean, and so was the bathroom. The hosts provided a convenient shuttle service to and from the airport. They even took me and two other guests for breakfast on the beach! Everything was a real treat. I would go back there in a heartbeat.
This is the perfect place if you are in Cagliari for business, conferences, a relaxing vacation, and if you prefer to stay a bit away from the noisy night life.