I find for technical books, paper is far superior. The layout is much better for graphics/ equations, plus the traditional layout of a book (with some post it stickies) is optimized for easy navigation/ jumping back and forth.
One thing I miss about paper books is their three-dimensionality. If you are an experienced reader, knowing where you are in the thickness of a book is just as important for navigating the book as a whole as where you are in any given page. And I don't think the GUI bookmarks work quite as well as paper, personally.
Really, it is all about fractions of a second, but it adds up.
(Personally, I don't care about the aesthetics, feel and smell of the paper, blah blah, I just still think paper technical books win for .)