Overall we enjoyed our stay quite a bit and would recommend the apartment to others.
The apartment is situated in a quiet residential area within walking distance of public transport (buses and metro) and close to nice restaurants, bars and shopping - not to mention the Vatican. We recommend walking to the "Mercato Trionfale" for fresh produce and a very authentic Italian experience (go in the morning!). There is a wonderfully tasty & very affordable bakery "Dolce Maniera" nearby - it's actually open 24/7! We loved everything we tasted there. "Osteria dell'Angelo" is a nice authentic Italian restaurant for lunch (à la carte) or dinner (3-course menu for ~24 €) - we went there twice.
The apartment itself is very peaceful and has an amazing view, a fully equipped kitchen, a wonderful balcony with a table and chairs; the bed is extremely comfortable and the ACs in the bedroom and kitchen work wonders on hot days. The shower is great (rain-shower with good water pressure!) and there's roof access for an even more impressive view!
There were a few minor issues that could easily be improved: soap and shampoo in the bathroom are included according to the listing but were missing. There's a fully functional washing machine which we were happy to use, but had to get our own washing powder. Lastly, the WiFi did not work well; we could only access it in the kitchen and even there it was very slow. It didn't matter much to us, thanks to free EU roaming; for non-EU travelers it could be quite annoying though.
We will definitely come to the wonderful city of Rome again!