Rated 4 out of 5 stars

I can confirm what Reiner told on April 14, 2015:
https://addons.mozilla.org/de/thunderbird/addon/thundersync/reviews/699270/

I tried to import from UTF-8 VCF file, but all umlauts got worse. Simple workaround for me: copy all files into a single VCF, convert this to ISO-8859-1 and import from this single file --> worked.

But it's not the right way to simply workaround existing UTF-8 coded files...