The apartment is awesome. It has a good mix old-world charm (furnishings in main bedroom, raked ceilings & front door), but is very modern in its renovation. Bathroom & kitchen are very good. Bed was comfortable. The pictures on Booking.com are representative.
The location is excellent - just out of the hustle & bustle of the tourist sites, but easily walkable to all. Is obviously close to the river & Vatican City, but we also walked to/from Piazza Navona, Trevi Fountain, the Forum, etc. Conveniently, it is around the corner from the hop-on-hop-of bus stop 6 (& other local bus stop).
The apartment is on the 3rd floor; we could happily live there. After a week in Cinque Terre, Amalfi Coast, & Capri, this was the best accommodation we had - a nice apartment.
The host, our contact, was Christian. He was both helpful & responsive. He advised this venture was relatively new for them (which was slightly evident, but was fine).
We were only there for two nights.
Overall, the stay was near-flawless and we would happily stay there again. And would be happy to recommend to friends.
Tips: We found the easiest way there was a taxi from Roma Centrale train station (approx €10), and Cristian organised a taxi/transfer to the airport for us. Maps - we used those from the hop-on-hop-off bus around the corner (and our smart phone)