Annalisa and Fabricio were magnificent hosts. They were extremely friendly and helpful, offering suggestions for restaurants and attractions from a local's point of view. Communication was fast and easy (as English-speaking guests, we had no problem at all).
The flat was extremely spacious, easily comfortable for my party of 4 (only 3 beds, so we shared, but two of the beds were large enough to share easily). Everything was clean and the air conditioning was sufficient to cool the entire flat very well. (We were there in mid-June, with temperatures in the mid-90s Fahrenheit).
The flat is very quiet, completely isolated from the sounds of the traffic and surrounding city. All of the windows face inward onto internal courtyards and the flat itself is toward the center of the building, so none of the walls are exterior walls exposed to the street.
The location is excellent. A bus stop and an entrance to the metro are just down the street, with the central Termini train station only 2 metro stops away. We walked 10-15 minutes to the Colosseum (or could have taken the metro there from the nearby metro stop) and also found a grocery store and a couple of nice restaurants within easy walking distance.
Two minor points for future guests:
1) The bath tub has a flexible hose and shower head, but there is no shower curtain around the tub so it's nearly impossible to shower standing up. The bath tub itself is quite nice however, including jacuzzi-like jets.
2) There were no paper towels in the kitchen and only a small supply of toilet paper (we purchased our own after the first 2 days). I'm confident Annalisa and Fabricio would have provided these if we had asked - I didn't want to bother them for something so minor that was easy for us to take care of.
We had a great experiencing staying with Annalisa and Fabricio and would definitely stay at their flat again if we make another trip to Rome.