After staying for a month in Yearn's apartment, I highly recommend it. It is a beautifully decorated flat, equipped with everything one needs for a long term, including an internet connection. Yearn really knows how to take care of the apartment, and it shows. It is located in a one of the loveliest areas of Berlin - in Prenzlauer Berg - and has an adorable balcony facing a charming courtyard.
The neighborhood itself is full of excellent restaurants of many kinds, and it is one of the only areas of Berlin that still keeps the appearance it had before the Second World War. The public transportation to the area is very convenient.
Yearn is a very attentive, nice and welcoming host. For example, I had to check our later than the scheduled hour, and he allowed me to stay for a couple of hours more.
He responded very quickly to each and every message/question that I had. And before I got there, he supplied a detailed directions.
All in all, I think this is one of the best options for accommodation in the city, and no doubt I will try to book it again next time I'm in Berlin.