The stay at Marius's place was great!
The building has some great history and Marius has a mid-century-slanted eclectic taste with his furnishings. We were travelling with our two young kids (2yrs and 4yrs) and found Marius's place perfect. We were all able to sleep comfortably in the bedroom, divided between the main bed and a comfortable floor mattress. The lounge room was large and light-filled during the day. The place was quiet and there was a washing machine. Unfortunately, there was no bath for the kids.
Marius's place is located in a grungy area with plenty of places to eat and vintage stores to visit (if you've ever visited Sydney, Australia it's similar to Redfern). There are some seemingly less desirable areas nearby that we walked through on the way from the train station, but we didn't have any trouble and there were plenty of families around with young kids (and plenty of playgrounds).
Marius was a great host - he made us feel welcome, provided extensive information about the area, and was quickly responsive to questions.
We would definitely recommend Marius's place to stay in Berlin!