We loved our stay in Testaccio. The location of this apartment is fantastic. It is outside of the historic center but is still a bustling Roman neighborhood. There are great restaurants in the area, and beautiful streets to walk through. We love walking and didn't find the 30-40 minute walk to the major sights to be a problem. In fact, this is our favorite way to see new cities. I definitely recommend walking up Aventino hill and visiting the gardens up there, as it's just a 10-15 minute walk from the apartment.
The apartment itself was very comfortable. Two of us shared the bedroom and one person slept on the couch. The kitchen was well-appointed, which was handy because the apartment is down the street from a great food market. The second bathroom (with a washing machine) was nice to have. The building was quiet and we were never bothered by street noise.