I stayed at Flavio's flat for 10 days, and I highly recommend it! Flavio was a great host - he accommodated my check-in time and responded quickly when I had a few issues. The location is perfect, right in the middle of the super-cool Testaccio neighborhood. It is an enjoyable walk to the Forum, Colosseum, or Trastevere. It is close to great supermarkets, restaurants, bakeries, coffee bars, shops, and public transportation.
As others mentioned, the flat is small but it is comfortable for one person even for an extended stay. It has large windows and a strong heating unit. The bed is a mattress on the floor of a raised ledge (as pictured). There is a separate shower room which is great so the bathroom does not get wet! There is also a washing machine and iron. The kitchen is small and has some quirks, but having the stove to cook was convenient. Flavio provided some basics such as salt, pepper, and a very delicious olive oil!