The apartment could not be more central--walking distance from all the major sites--Vatican, Trevi Fountain, Pantheon. And just steps from Piazza Navona.
The apartment itself is really big--great for a family. It also has everything you need to cook, if you'd like. The only downside is that you need to walk 4 flights of stairs to get to the apartment--tough if you're at all mobility challenged.
One other thing: the day we arrived there was a big thunderstorm and the power was out in the apartment for the first 24 hours. I guess this is what you get with old historic buildings. Flavia and her team kept in close touch, though. And offered croissants and cappuccinos as recompense. :)