I check books out of the library and read them all the time. The library I use has books in Mobipocket and PDF format. I read the Mobipocket books on my Cybook and I read the PDF books on my Pocket PC (I can also read the Mobipocket books on my Pocket PC)
Both types of books have the same kind of "time bomb" DRM on them - after 21 days the books will not open. I had to activate my Pocket PC through Adobe's website before I could read the PDF books on it. The Pocket PC handles the PDF books very well because the books are all tagged, which means they reflow to fit the Pocket PC's screen size.
The Cybook does not handle DRM for PDF formatted books, nor does it recognized tagged PDF files, so no PDF files reflow to fit the Cybook screen.