We stayed for 4 days at Casa del Cinque and had the best time there. The driver recommended by Margrethe picked us up at the airport, check-in/check-out was quick and easy, everything (gas, heating, water etc.) in perfect working order. The apartment is just as lovely as the photos suggest, the views are breathtaking and we had lovely mornings on the sunny terrace. We didn´t eat in, but the kitchen provided everything needed for cooking and the assortment of "useful stuff" from adapters to candles is very thoughtful. I´d like to point out the perfect location of Casa del Cinque: we managed to make all our trips across Rome on foot! Trastevere with its many occasions for eating out turned out to be the perfect starting/ending point after a long day of seeing the sights.
Thanks to Margrethe for providing all sorts of useful information and your quick answers! We will be back!