Great location! Testaccio is definitely very local which we loved! [We highly dis-advise using buses in Rome (they're very unreliable when e.g. getting from the airport), maybe splurge on a taxi if possible...]. If you're a fan of getting to know the city while walking then this is a great starting point!
The windows of the apartment are facing a little courtyard and small street (which at night was very quiet and peaceful), so you can easily relax after a whole day of sightseeing.
Right outside the entrance to the building there is a small Frutta & Verdura store where you can get something for a snack, water for the road or a bottle of wine. Although, be aware that there aren't any 24/7 places around if you need a late night snack! :)
On the other side of the street there is a small cafe - great for starting your day with a coffee and pastry.