I spent 4 months at Mariposa in 2017, and found the entire experience delightful. The apartment is bright, airy, modern, and nicely decorated, with great beds. The bedrooms are spacious and well appointed. The bathroom features a lovely, large, glassed-in shower--much nicer than most Italian apartment and hotels. While the kitchen and living area are small, I was very comfortable there, even with guests.
Marina and Francesca are the best hosts I could imagine: friendly, welcoming, responsive, and willing to help with all sorts of small problems. They live in the next building, and whenever I needed help--when I left my keys at the gym and couldn't get in to the apartment, when I cut myself with a kitchen knife and the pharmacies were all closed, when I needed to find a doctor to get a swimming permit--they were right on the spot and willing to help.
The apartment also has a wonderful location. It is a real Roman neighborhood, outside the historic center and so not full of tourists. But the city center is still easily and quickly accessible, with a nearby Metro stop (8 minutes to the Coliseum), several convenient buses (20 minutes to the center), or a very pleasant walk (about 45-50 minutes). I can't imagine a place where I would have been happier.