I have much to say, but will try to be brief.
First, the apartment is just as advertised. It is a typical Berlin floor plan with main hallway, kitchen, bedroom, and living room (pull out couch) coming off of the hallway. It is not a fully lived-in apartment and appears to be used for (only) rentals. No worries, it feels very cosy and well maintained (there are little touches, most furniture is from IKEA and that is OK because this is typical of all Berlin apartments!). (One note, pay attention to the final price, because adding people more than 2 adds to the price significantly)
The apartment is SUPER quiet; you hear nothing from the street or from neighbors.
The apartment is on a dead-end street so there is little street traffic and you are on the inside of the courtyard space, so it is very very quiet.
That said, it is not in the center of the city, so you are not in the crazy active area of things. This was good for us. There is just enough activity at night that you feel at home (Gleimstr. has some nice restaurants with outdoor seating, keeping it lively). The Mauerpark is a bit rough, it is both a pace that is good for families (the farm in the park is fantastic for kids who love animals) and at the same time a place that feels a little peculiar. But this is also the feeling of Berlin and of East Berlin in particular. In short, we stayed here for 3 weeks in August as a family of 4, and had a wonderful time!