The location can't be beat and the value was fantastic.
It's directly on the RER line from CDG (less than a block from the stop), so coming and going, you don't need to make any transfers, which is really convenient. The Jardin du Luxembourg is directly across the street - perfect for a run, or just relaxing and people-watching with a coffee. It's an easy and scenic walk down Boulevard Saint-Michel to the historic city center. The apartment itself is a bit sparse perhaps, by American standards, but if you're there for the city (we were), you won't be spending tons of time there. As a few other reviews have mentioned, the apartment can also get warm - there's no AC or fan. While opening the windows provides a lovely breeze, some may be bothered by the sounds of traffic below. Finally, Marie was lovely - very responsive and helpful. We thoroughly enjoyed our stay.