The apartment, despite being so close to St. Michel (a popular tourist "spot") was lovely, on the Place St. Andre des Arts, and very convenient to the metro and RER as well as many of the main attractions of Paris. We took the RER from CDG to St. Michel-Notre Dame and walked just one block to the apartment - what a luxury! (The RER starts from Term2G I believe; we had a 5 minute walk from Term 2B where we had landed.) As we were too early for check in, we went to the Musee de Cluny (the medieval museum located on a site of ancient Roman baths) 1 block away and had a cafe nearby.
The apartment is on the 3rd floor up some very narrow stairs (I imagine the building was built centuries ago), so if you have large or heavy luggage it will be a workout. It was clean and spacious enough for us... though we are experienced European travelers. The kitchen was very simple and would have sufficed for basic meals but we stopped at the grocery store and picked up a picnic for a park most days, and enjoyed the local restaurants otherwise for dinner. There is a small washer/dryer combo. There is no HVAC but September in Paris does not require it truly, and because there is a window on two sides the cross breeze was great. I honestly expected more noise given the proximity to St. Michel; it was surprisingly quiet except for the garbage trucks in the am (which you can't avoid anywhere in Paris).
Above all, the location was the best - a 5 minute or less walk to the Jardin de Lux, Pantheon, St. Sulpice, St. Germain, Notre Dame, the Ste. Chapelle (go to a concert there!), a quick jaunt across the Ile de la Cite to the Hotel de Ville, Marais, Centre Pompidou, and not much farther to the Place des Vosges. A bit farther for the Louvre and d'Orsay, but very convenient if you want to take the metro (note: the Metro St. Michel and St. Michel-Notre Dame are different - make sure you get that correct!) And the RER at St. Michel-Notre Dame also goes straight to Versailles if you are planning that. Rue de la Harpe and rue de la Huchette nearby have budget eateries; you can go in the opposite direction to rue de Buci for some other lively and lovely street restaurants.