Carlo's place was excellent. He responded very quickly and helpfully to all pre-travel emails. A very helpful guy named Alessandro met us at the apartment and got us situated: showed us how things worked, gave us a map and suggested we get a taxi to the airport for the way home, then arranged it for us. That was a great tip, as the direct taxi ride was only about 10 euro more than taxi and train combo would have been, and it was stress-free.
The apartment was excellent. Very comfortable and spacious, and had everything we needed (hair dryer, coffee-making options, toaster, etc.) It's very well located - we walked to everything (Colosseum, St Peters, etc). It was great to have a place in the middle of everything so we could get some mid-day down time if we wanted.
Some reviews mention that there is nighttime street noise. We did hear people on the street most of the night with the windows closed but it generally didn't prevent us from sleeping (and earplugs would probably solve the problem).
One thing that could be improved is for the bed to have a pillow-top mattress cover, as we could both feel the springs in the mattress.