We had a great experience at Maria Cristina's.
The other reviews we read were very good, and we agree.
Maria Cristina was very helpful, quickly replying to our queries.
The apartment is nice, and has air conditioning in the bedroom and living room which were great in this very hot Roman summer.
There's an app on the lock, so we were able to let ourselves in for our 2nd stay without needing to meet Maria Cristina in person the 2nd time.
Testaccio is a great neighborhood. Central without being too touristy, and has a great selection of restaurants.
We would definitely stay here again.