The apartment itself was great--spacious, with almost everything you'd need, the heating worked well, and endowed with 1.5 bathrooms (very convenient!). The neighborhood (Quartieri Spagnoli) is everything you'd expect from Naples: lively, eventful, cobbled streets, lots of people, laundry hanging across the alleys, plenty of restaurants and street food, and so forth. But it was was also noisy at all times of day and night, crowded, and a bit overwhelming. Most importantly, reaching the place by car (a station wagon, no less) was difficult, and leaving again to get to the paid parking I'd booked was a nightmare. Best not to come here by car!
Mario was a graceful host. He was there punctually waiting for us, helped us maneuver the car in and out, was always contactable and ready to provide advice.