This is a beautiful name and I would definitely consider giving it to a daughter to honor my Polish heritage, or use it as a nickname for a girl named Sophia or Zofia. It's not too common from what I understand, and in America it would sound interesting and a little exotic but not too foreign or weird because English speakers are already familiar with the name Sophia.
In the novel Sophie's Choice by William Styron (1979), Zofia (Sophie) is a Polish-Catholic survivor of the Auschwitz concentration camp, and she goes by Zosia to her close family and friends in Poland.
The Polish tongue/język (as I have studied it) is remarkably more consistent in its pronunciation and its grammatical rules than English. As such, all "o" vowels in Polish words should be spoken with a long oh (boat, coke, oar) pronunciation--not the "AW" (bought, hot, drawn) sound shown here.
It's actually the other way round. Polish language has indeed very simple rules about how to read words. But the letter "o" in Polish language is pronounced as in the word "bought" not "boat". The phonetic transcription to Polish of these two words would be accordingly "bOt" and "bOŁt". For us -Polish speakers- if someone says "oh" (as in "oh, it's so nice") it sounds like "oł" not "o". Also, if we're already on the subject, "sia" isn't exactly pronounced as "sha" - in Polish this sound would look like this: "sza". "Sia" is a soft sound. More like "xia" in pinyin transcription of Chinese. But for this site "sha" appears to be the closest we can get.
And about the name itself, I find name Zofia really beautiful. And it has a beautiful diminutive.