Is this word really much used in German? I don't even have it in my dictionary.
I'd say it's fairly common.
dict.cc says that it is colloquial.
Colloquial, but yes @christian, very, very common.
dict.leo.org offers another very good translation: "it is frantically busy." "Stressig" is a word we use for something or an environment that will be under high pressure and very demanding.
english is not my native language, but why is "so" instead of "very" wrong in this case?
Maybe because "sehr" is "very" and not "so". "So" is "so" in German (pronounced like "zoh").