I have a PRS-600; I also use Calibre as my interface software. I do have the Sony reader software running, but the Sync function is useless. I only use the Sony software for books purchased from the Sony store. I get other books from Kobo and Smashwords; I've acquired a few others from authors' webpages. You can find other free e-books using Inkmesh (
http://inkmesh.com/free-ebooks/ ). That one even allows you to sort by which stores you shop.
Enjoy your 600!