I stayed at Annika's place for three months for a business trip. The apartment is lovely with a very nice view in the river. It is fully equipped so nothing for me to buy. The apartment is very nicely decorated and has a lot of storage that is not visible in the photos.
It is close to 2 different train stations, 4-5 min walk and very close to a supermarket, pharmacy, bakery etc. It is a residential area, quite and family friendly. It is also very close to the city center and it is practically in the middle of Berlin.
Annika and Johannes reply immediately to emails and they have been really helpful and heartwarming hosts. Definitely the place to stay in Berlin.