I have not tried Calibre yet, one of these days I probably should.
Most of the books I have are public domain stuff that comes in either .lit, pdf or a format that jetbook supports.
While the Jetbook does support Pdf files, I convert mine to straight text files as I don't like scrolling for every page. PDF's that have a lot of images might be worth the scrolling however.
The 2 converters I use are ABC Amber Lit converter, and
Zilla PDF to text converter. Both free, both do a decent job.
Not fabulous, but for the price, certainly worth the download.
Its probably faster to load books onto an SD or SDHC card with a card reader or adapter.
Jetbook will do it, but its slower.
I would say I use .rtf and plain .txt the most, but that will vary widely I suspect between users.
I think a lot depends on where you want to get your books. The support for Epub and drm is going to bring a lot of new people to jetbook I think. Granted it doesn't have wireless but your not paying for it either.