When I tried importing an RIS file from endnote X, (and I tried some other types) zotero would come up with with an importing progress bar, but indefinitely. After waiting some time (like, hours) I would have to restart firefox, and found that zotero had imported all my references starting with authors with B as a surname, and lots of references as web pages with some details but missing the reference. I tried a few times and zotero only seems like "B" people. ..