Great experience overall and an amazing place to stay.
The flat is beautiful, very well-styled as well as practical (this was quite a relief - particularly in that the pictures on Airbnb were very accurate - as we were just tired of holidays in Italy in very tired-looking, old-school or just plain ugly properties).
Location is amazing - about bang in the middle of everything you would want to visit - 5 mins walk to Piazza Navona, 10 mins to Pantheon or Spanish Steps, 20-25 mins to Vatican and Colosseum.
The hosts were great - Fabio met us in person and was lovely, and they have a very useful booklet with local recommendations etc. and also can help with taxi pick-ups etc! (We did not try the fast train but if you are looking for taxis would recommend - Ubers are somehow more expensive in Italy and the pick up experience is worth it - taxis from FCO were €50).
One warning for those less able is that the flat is on the 3rd floor so there are some stairs to walk but it is not too bad.