I had a wonderful stay at Federica's. She is a fantastic person and ready to help should you ask. The room is great, having a beautiful view of Capri from the balcony. There are small stores and restaurants within a block of the flat. WiFi is solid. Premises is very neat and clean, and the neighborhood is safe.
I will mention the metro stations are a walk. There is a bus stop in front of the building but service is probably hourly.
I highly recommend staying with Federica when you visit Naples!