A friend and I stayed for 9 days. All in all, it was a very pleasant stay and we were very happy with the price and the rooms.
Unfortunately, the metro was under maintenance or construction and did not work. Therefore, we had to walk a lot. Since the building is located in Materdei, which is based on a hill, it might be quite exhausting for people who aren't that fit. It takes about 30–40 minutes to walk to the center.
The bedroom was in a very good condition. The beds were comfy. There is a hair dryer, a well working air conditioner and much space for storage. The room was freshly renovated, so the photos here a little bit misleading (The room is green now).
The bathroom is very spacious and can be locked, so you can store all your items in there.
The building itself is in a very good condition as well. There is an elevator which we absolutely loved. A supermarket is right around the corner and there's also many small shops where you can buy drinks and snacks. Materdei is quite dirty though (actually, Naples in general is very dirty as you might know); watch out for dog poop!
We have never met Marco personally but he was always very fast in replying and very polite and helpful.
In conclusion, I can definitely recommend this apartment.