We had the most wonderful stay at Carla's Studio in Campo de Fiori. The place was clean and cosy. The bed was really good and besides for some occasionally noise from the streets during night, we slept like babies. The studio is in walking distance to everything important in the city center. Minutes from Campo de Fiori and Piazza Navonna and only 10-15 minutes from the Vatican. In the neighborhood you find nice restaurants and bars too, but only a 20 minutes walk and you are in Trastevere with a lot of restaurants and bars.
When we arrived we were welcomed by a friend of Carla's and got a friendly tour of the place and lots of good information about the area. We can only recommend this place.