Fantastic location! We walked everywhere, from Vatican City to the colosseum. We didn't need to use public transportation once, and the only time we took a taxi was the day we left.
Great hosts. They arranged a private transfer from the airport -- extremely helpful since we were bleary-eyed from overseas travel. Mario checked us in, and was very personable, offering good tips. Wine and snacks were left for us, along with written information about the apartment and the area.
Wi-fi didn't work for us in the bedrooms, and I can't argue with other comments about the tiny elevator and slightly worn furniture. But I thought the place had character, and you cannot beat the location. I would definitely stay here again.