I don't use torrent sites for my books (novels), but the ones I do get over the net in other formats, I generally use Stanza to convert to epub.
Sony Reader store uses ePub. My local library (Overdrive supported) uses ePub. I believe the books bought from BN for Nook also use ePub.
The big difference between ePub and PDF is reflow capability. An ePub reflows much nicer -- providing for different fonts, different sizes, etc. PDF's don't reflow so well, but they're better for tech books, flight manuals, programming books, etc because of the image/diagram support.
I'm still stuck w/PDF if I want to study, highlight, annotate, etc. But I use ePub for anything that's good-old "pleasure reading" -- like novels.