The problem i see in the movie is with the casting: the actress that plays Sonja is just too normal, with model-like looks, whereas the author of the novel was much more peculiar. "Einsam in Berlin" -- an Italian, with rather voluptuous looks, she must have felt a bit isolated in Berlin, which eventually lead to the lifestyle. I think there is a disconnect because of the casting. Even if the book material might have not been very dramatic (being a biography, not a story), the movie is actually very watchable. The soundtrack, the city are beautiful, you see a snapshot of everyday berlin for younger, poorer people.
It's really a story about a lonely woman in berlin, during the age of legal prostitution and as berlin becomes a meltpot of its own. The layer of people that she meets might be very specific, but they are out there: the immigrants, the drug addicts, people from poorer regions. Perhaps the author wanted to separate her 'normal life' from her 'prostitution life' -- so we don't see much of interaction with her friends at school, or, perhaps, the author was autistic enough to be in a more isolated position, and the 'prostitution' life filled the friendless void.
But in essence, the movie gives a different angle on a woman and a city, not that glamorous as 'Sex & City', but there are layers in the society, and the movie captured one of them. The layers are different, the upshots of the stories are very similar.