I stayed at the apartment for 2 weeks.. It was a nice, cute apartment in the 18th department of Paris, 15min walk away from sacre coeur and monmarte.. It isn't a very quiet neighbourhood but it didn't disturb me at all..
The metro station (line 4) is a 3min walk distance.
The disadvantages were the wifi connection (which after communication with karl got fixed) and the absence of an elevator (3rd floor) .
Karl was very helpful with whatever i asked and provided solutions every time..
I totally recommend it..