Location, location, location! You cannot beat this location. Everything is within walking distance -- and that is essential in Palermo unless you want to spend a lot of money on taxis (they are expensive) or a lot of time on buses (which tend to be very crowded and often require long waits). Not only are the must-see sites all within easy walking distance but food, drink and anything else you might think of are also steps away. The building is also very safe, with a concierge on duty twice a day.
I stayed for a month. While I don’t think the apartment, which has the feel of a hotel suite, is actually set up for long-term stays (no change of sheets, though you can pay to have them laundered; or just do them yourself in the washer and hang to dry first thing in the morning) no real kitchen counter space, limited cookware, and no closets in the bedroom (just an open rack with 6 hangers), I would return. Who wants to spend lots of time cooking when every kind of food and drink, from coffee to chickpea fritters to fish to whatever Sicilian food you could dream of right outside your door.
More good stuff: The place was spotless, Bernardo could not have been more responsive, the mattress is comfortable, the kitchen stove has four burners, and wifi is reliable. (I have stayed in about 15 AirBnbs this past year and this was the only place that did not have a single wifi burp.)
The terrace is lovely -- astounding views -- and is accessible from both the bedroom and the living area. There is a small table with two chairs out there. (I wished there was a lounge chair. Maybe they will put one there now that I suggested it!)
Thank you, Bernardo!