We had a nice time in the flat. The location is great, close to everything that matters. On arrival, we bought 8 bus tickets, which we never used as we walked to the Spanish steps, Piazza Navona, Fontana di Trevi , Colosseum (20 min min), the Vatican museum (25 min), etc.; we even walked from the Colosseum to the Termini railway station for 25 min.
The check-in is a bit late (15:00), but there is a Stow Your Luggage storage just a couple of hundred meters from the apartment (you should book in advance on their website or the cells may be all taken).
Just below the apartment there is a cafe which opens at 7:30 or 8:00. The coffee and croissants (clearly better than in Paris) are amazing. You can ask your croissants to be filled with cream and they do it on the spot.
The flat is close also to the shopping area.
The apartment is stylish, clean and very quiet despite the central location; it has two nice rooms and a small kitchen. The bathrooms are clean; one of them has a big sink, a toilet and bidet and the other one has a small shower cabin and a sink and a toilet. All clean.
Finally, the area is safe.
So, when we go back, we will certainly go back.