We loved our stay in Via Casilina. In addition of being very clean and comfortable, Francesca's and Leonardo's apartment is full of character, due to vintage furniture, beautiful paintings and a great library featuring books ranging from art and fiction to geography, history and philosophy. The location is very close to the bus and tram stops and therefore well connected to the centre and the main railway station (Roma Termini). (Quick tip for whoever spends 7 days or so in Rome: we recommend buying the 7 day travel card which costs €24 and allows unlimited transport by bus, tram and subway. There are also some passes for both travel and museums.)
Francesca and Leonardo were very kind to offer us a bottle of wine for the New Year's Eve. They were also very prompt and responsive when we brought to their attention some small issues (not their fault).
Bottom line, staying in Leonardo's apartment was a great experience and we wholeheartedly recommend it to other travellers.