One thing I like about paper books is the tactile aspect. Each book is unique, feels differently, has it's own touch (and smells differently). A device physically always feels the same, regardless of the book read. I recall seeing a study that indicated that for this reason it's also easier (for children) to learn from physical books and retain the material better. There is more "information" for the brain than the content. I think this may be true for me, but it's difficult to say with certainty as I tend to get physical books only (if there is a choice) if I'm really into the subject matter, so I am also more likely to engage more deeply with it.
Another pro for physical books (and to a slightly lesser degree: e-ink readers) is the "isolated island" factor. When I read on a tablet or phone, I am far more likely to multitask. I'll check the forum, Wikipedia, mail, work stuff, etc. if at the press of a button I can switch to a browser or (worse) messaging program. It's different if I have to put aside a book and pick up the tablet or walk to the computer. As a result, I'm more concentrated and focused when reading paper books. This may be related to better retention of the content, too.