We spent 4 nights at Sabrina's B&B and we were totally happy. Sabrina is very nice and the room was super clean. The breakfast is typical italian (espresso/cappuccino and croissant) and delicious! The location is ok, it's not very close to the main attractions but you can either walk and enjoy the beautiful Trastevere or just take the tram (cheap and leaves every 5 min).
So we would absolutely recommend spending a few nights at Sabrina's B&B La Lupa :)