We stayed at this apartment from May 25th to 1 June 2013, with the kids, (8 and 10).
It was a wonderfully quiet place, just metres from the main drags, a supermarket, several restaurants, and only a stones throw from the Spanish steps, the trevi fountain, plaza narvona and the main shopping areas. We only needed to us the metro, to get there. Once there its within 40 minutes walking distance of everything, including the vatican, and the colluseum, (map from the owner)
The best thing was the park, just above the plaza de polopo, which was 2 minutes away. two of the evenings we hired bikes there. Great fun.