We (2 adults and 2 small kids) had a great stay in the apartment. It looks just as shown in the pictures and being located through a courtyard, remains quiet despite being in a busy part of the city. The location is perfect - virtually all of the major attractions are walkable or an easy metro/bus journey away (bus direct to Louvre leaves from right outside the door).
The apartment is on the ground floor which means it is easily accessible. The space is a good size (by Parisian standards) for a family of 4 and it was great to have a separate bedroom for the kids. We were staying during a heatwave when the hottest ever temperature (43C!) was recorded in Paris so having the air conditioning unit in the apartment made a lot of difference to our enjoyment of the trip. We were able to go out in the morning when it was a bit cooler to see some sights, come back and relax and cool down for a while, before heading out again later. A definite plus for the warmer months!
A selection of up to date Paris guidebooks is available in the apartment which is a nice touch. Although we didn't have any questions, Frederic was proactive and provided clear instructions regarding check-in so I'm sure he would been responsive to any questions. It was all made very easy though.
Overall a great apartment in a great location and at a good price - looking forward to coming back!