We thoroughly enjoyed our stay in Jean-Noel's flat. It's in an amazingly vibrant part of town. Not only do you get a great view of Notre Dame out the window, you can find just about any amenity you'd like within .5k (boulangeries, grocery store, even a near-daily produce/craft market fronting a fromagerie and a charcuterie right next to the Maubert - Mutualité metro station). The flat provides very easy access to the city via the metro; also, the Saint-Michel Notre-Dame stop (which will take you farther out to Versailles, CDG airport, Disney...) is less than a 1k walk.
However, be sure you are looking for that type of experience (busy, vibrant) when booking this flat, at least during the summer months. During our June stay, street musicians played nightly right below the windows, often running to midnight. The music was generally lovely and not amplified, and complemented the stirrings of the restaurants below. It did not disturb our kids in the back bedroom at all and we loved it. But if you like to tuck in early, bring a sound machine or headphones. The windows do a pretty good job quieting the noise when closed; however, during the summer you'll want to leave them open for airflow. Also, the stairs will be difficult if you have balance or mobility issues.
The next time we make it to Paris, I hope this flat is available for I would surely book it. I honestly was blown away and Jean-Noel's flat was the perfect platform for our fantastic time in Paris.