We had a fantastic stay at Emanuele's Oppio Boutique B&B. The private suite was more spacious than the pics indicated, and it has the basic amenities (tv, a/c, mini fridge) of any American hotel. Tastefully, simply decorated. We really loved it and our time there.
We were there 6 nights and never noticed the adjacent occupied units. Emanuele took great care of us from the time of booking to our arrival in Rome, arranging to meet us and guide us to the building and showing us to the unit. He was friendly without being overbearing or in our space all the time. He gave great recommendations for restaurants, tips for getting around town and so forth.
The location itself is ideal if you want to explore the Coliseum and Forum area. You're walkable to the main Metro line (for most folks barring physical issues) and train line (for city dwellers and younger folks; a cab ride for older folks). The area felt safe, filled with locals doing their thing (even if you will have other tourists like you around). Several grocery stores around to economize on snacks. You feel what it would be like to live in Rome in this neighborhood.
You can't go wrong here!