We highly recommend this apartment for a week or less if you are looking for a great home base to explore Rome.
NEIGHBORHOOD: Testaccio is more relaxed than the more touristy areas near it, but still convenient with several great dining and grocery store options. If you don't know already, this is Rome's "foodie neighborhood," which is really saying something in Italy! In addition to walking places, we used the extremely affordable bus system and found that there were many useful routes that connected to the neighborhood. As others noted, it was nice to regularly run along the Tiber River.
APARTMENT: The apartment itself is tastefully decorated and comfortable for two people. The trees just outside the windows are quite beautiful with the sun dappling through them. The bed, bathroom, and kitchen are all efficient enough with the amenities needed. The large closet turned out be extra helpful for our longer stay. The living room is spacious and comfortable enough for unwinding at the end of the day. Admittedly, the plumbing is what you expect from an old building. We've stayed in a half dozen AirBnBs in Italy and about 1/3 of them have had some kind of issue--so I know this is just par for the course with living in these very charming buildings. As someone else noted, this may not be a great option for people with mobility issues due to the stairs--but we found them totally manageable.
HOSTS: Flavio and Emiliano are fabulous--very warm, responsive, and helpful. They arranged car service to/from the airport.