We (group of friends) enjoyed the apartment from Frank and Christa very much!
It is in the middle of the beautiful Marais and everything is close-by; renting bikes, metro St. Paul, all the nice little streets full of shops and boutiques, restaurants and terraces... And still it is quiet in the night.
The apartment has everything you need for a stay in Paris; good beds, enough space to chill and a good shower :-). Also, we could park our car for free in front of the main door!
Frank and Christa were very friendly and helpful, we would certainly recommend their place to anyone who wants to live in the middle of the Marais for a few days!