We loved the apartment, it is very modern, immaculately decorated and spotlessly clean. It is also well proportioned with spacious rooms, high ceilings, and benefits from air conditioning. There was no sound from the neighbours, or any road noise - it was a very calm and peaceful place to be.
Carlo was very responsive in answering our questions and helpful when our already later flight was delayed - a good host!
I can also recommend the location - not in the centre but in a quiet neighbourhood about three minutes walk from the train station (one stop from St Peter's and Trastavere). It's away from the crowds in the centre (but still within a reasonable walk) and the restaurants and shops are proper local places - just down the steps at the end of the street is a convenience store and an excellent bakery, just round the corner from these, next to a music shop, is a very good restaurant - we tried pizza, pasta and meat dishes and all were good. I'd also recommend a local pizzeria we visited, Cerchi Fabio.
I highly recommend the apartment, Carlo and Rome and look forward to going back some day.