First off - I cannot say enough about how wonderful, warm, and welcoming Hélène and her family were to me - a highlight of travel for me is the chance to make connections with folks around the world, see how they think, see how they approach life - and it was my great fortune to have a chance to meet and interact with Hélène and her family throughout my trip. The apartment could have been rubbish and I would still have loved the stay.
But .. the apartment was in fact fantastic - note that the loft bed will not work for everyone, but was great for me, including a skylight right above the bed that included a fully functional dark shade to allow sleeping in in the morning, and an amazing view of Notre Dame, the Left Bank, Ile de la Cite, and Ile Saint Louis. The pictures I got from that window - both day and night - were standout.
The interior was homey - not "swank", but very comfortable, and I loved it. The apartment was on the top floor, but they have an elevator, so no worries if you're accompanied by heavy suitcases.
The location was ridiculous. Essentially 30 seconds from Notre Dame, 30s from Ile Saint Louis, 2 minutes from the metro, 1-5 minutes to any of a number of really great and beautiful things to check out ... I don't see how you could possibly improve on the location.
No question about where I would stay next time in Paris!