"- Як справи? - Так собі."
Translation:- How are you? - So so.
I live in California. We do hear it. We probably did not travel in the same circles. The expression has even appeared on TV. Yes, I doubt that you would hear it in New England. I have a feeling that more people there would be among those that say fine most of the time. As in How are you? Fine. move the conversation away from the personal. It is part of a greeting, back and forth or do you really answer the personal question? For some people they say they are well or not well and are done with it. There is even a rap song titled So So by B.o.B. I would have posted a link, but the language is not appropriate for a family site like this. very explicit. I did not live in downtown L.A. or Oakland. I lived in the suburbs, but growing up, I heard other kids use it all the time. To say it is a word for second language learners would be wrong. It is an informal word, but it is listed in both American and British dictionaries. It has been around since the 1520's. and yet appears in a modern song. http://www.etymonline.com/index.php?allowed_in_frame=0=so-so
"So so" is not as good as "fine". It is right in the middle between good and bad. There are people who will just say fine, no matter how they feel. Others will actually tell you how they feel, "Great!", "Very well", "Fine" or "Good" or "Well", "So so", "Not so good", "Not well" or "Poorly" or "Badly", "Awful" or "Dreadful" or you name it. There are many adjectives and adverbs to choose from. Also, some people will give some indication, but will upgrade. Those people could be having the worst day of their lives, but won't go lower than "so so" or things could be "so so" and they will say "fine", things could be good and they will say "great".
It seems there are different uses of the phrase so so in English. In my experience, I don't recall having heard it used as a reply to how are you? It is commonly used when asking about an event: for instance; What was the film like?; How did the performance go? What did you think of the meal? So-so.