We really enjoyed our stay in this apartment. It was as described and clean and comfortable enough for us. Location is great in Le Marias and Metro just down the street. We didn't get to meet Yannick but he was great with communication and we had no problem following his instructions.
Apartment is on the top (6th) floor so there is lots of stairs to climb, but the views from the apartment are worth it...plus you can work off those extra pastries and bottles of wine ;)
There are a couple of small things I could mention like no wine glasses and cramped shower, and Wifi didn't work for us, but this isn't a hotel and you are getting a cheap stay in Paris. A great budget alternative and much more comfortable than a hostel.