We (a family of four) stayed at Piera's apartment for 6 days and had great time exploring Rome as everything was near and accessible on foot. We only used subway to go to Vatican. For those travelling by car: the house itself is out of the ZTL zone, so driving to Termini and then turning on Cavour will do the trick of avoiding it.
4 people fit in perfectly on 2 beds. You also get an air-conditioned 1 room apartment with 2 small balconies and a kitchen corner. Just don't take a lot of luggage with you, because the 3rd floor has to be reached by foot or have a strong man in your team:)
Piera was always reachable by phone/sms/email - a wonderful host!
All in all, I would recommend staying here and would stay here again myself, because the location, the apartment itself and the host - all provide great experience!