We stayed in Maria Pia's place for the new year's holidays and we cannot recommend it enough. We were 2 adult couples and the accommodations were perfect: 2 ample bedrooms that were very clean, comfortable and tastefully decorated.
The location is truly excellent. We were literally a 10 minute walk away from the Vatican, 15 from the historical center and 20 from the Trastevere restaurants and shops. The area itself is surrounded by a lot of restaurants, many of them well ranked by tip advisor; and a market open 24/7. Public transportation is also very easily available in case you don't want to walk.
On top of the excellent accommodation and location, Luciano and Pia were always available for consultation (they live upstairs) and even helped us hire a van from and to the airport.
Maria Pia's place will definitively be our first option next time we stop by Rome.