That was an awesome stay! Federico was very hospitable, and the apartment was exactly as in the pictures.
The location is excellent - close to the Vatican and very close to a metro (and bus) station which is extremely convenient if you want to go to other places in the city (besides he Vatican which is - as I said - close by). We didn't use the metro but we did use the bus a couple of times. There is also a supermarket just 200 meters away.
The only bad experience we had - which has nothing to do with the apartment, it was just bad luck - was that during one of he nights the neighbours next door had a party :-(
Overall, an excellent experience! Thanks a lot Federico!