I've been buying and reading all my multiformat ebooks from fictionwise. Generally jBook has no problem with epub format from fictionwise.
But a caveat though. My jBook has encountered problems with more than a couple ebooks from fictionwise (in epub format) that were published at least 4 years ago. Some magazine back issues and old novels couldn't be read at all, and a few look ugly on jBook because of formatting problems.
My iRiver Story has zero problem with anything from fictionwise.
Fictionwise seems to have remedied the situation. Most recent fictionwise ebooks seem to work fine with my jBook.