I like ebook versions of RPG texts, which are unfortunately as rare as hen's teeth. PDF is… fine. Not great. Pretty much unreadable on small screens (I sometimes have my phone's pdf viewer read them aloud while I drive, which is … differently sub-optimal), while larger screens are less easy to carry around.
Paper books are the clear winner for any book I plan to reference a lot in play, because it's still too difficult to flip to another part of a text while 'holding your finger' somewhere else in the book.
Bonus Content: I wrote about how to make e-readers suck less as reference tools three and a half years ago, with pictures – http://doycetesterman.com/index.php/2013/01/e-readers-suck-for-reference-materials-but-they-dont-have-to/
On the other hand, if I'm just reading something, electronic is fine.
I just wish we could get away from PDFs and more toward ebook formats.