My wife and I stayed in Christian's place for 9 nights in October. The apartment is compact, but well equipped. The location is fantastic - good/great restaurants around the corner, convenience grocery store a few blocks away and easy walks to transit stations. The Pont Marie Metro stop is a 5 minute walk and all the bus stations we needed were 3-6 minutes away. The apartment faces a small courtyard and there was no street noise at all.
Check-in and checkout were painless - key code to access the courtyard from the street and the apartment key in a coded lockbox. Christian is very friendly and an excellent host. We had one issue the first day - the shower barely drained at all. We mentioned this to Christian right away, and the next day the issue was completely fixed.
We are in our mid-60s with a few arthritic joint issues and were a little concerned about the tight circular steps up to the sleeping area, but it turned out to be a non-issue for us. The bathroom layout is compact and does not offer a lot of privacy, so be aware of that if your group is more than two people.
We would definitely stay here again.