Catherine's flat is a beautiful gem tucked in one of Paris's best (if not the best) neighborhoods. We absolutely loved our stay! The apartment had everything we needed to be comfortably (including a fan Catherine graciously dropped off to help combat the heat) and I would return in a heartbeat. We were within walking distance to every site we wanted to see and being 5 minutes from the Seine was absolutely wonderful.
The only thing I would keep in mind is the no AC and the several flights of stairs needed to get to the top :)