We have stayed at Happy Navona for a week and it was a lovely experience. The apartment is very cute and perfect for a couple. The location is excellent - a 3 minutes walk from Piazza Navona. We reached everywhere by foot, including the Colosseum and Vatican.
Lucilla is incredibly nice. She made cheap taxi reservation for us from and to the airport, she welcomed us at the apartment when we arrived with great local pastries and gave us detailed explanations about the neighborhood. Lucilla also kept in touch with us throughout our stay to make sure everything is alright.
Overall we will definitely recommend this place, especially for couples :)