I prefer e-readers (Sony Reader, Kindle DX, iPad) because I read at the speed of about one airport novel per hour, meaning I need five books for a cross country flight. It's easier to carry these electronically, and I keep a backlog of 50 - 70 books available to read.
For computer books, such as the jQuery Cookbook I purchased yesterday from O'Reilly in this sale, I will also read each one from cover to cover. With that reading I form a visual spatial memory of where in the book I can find any information I need.
My memory is not eidetic. I can't read the actual words, and I don't remember every page number. But I do know about how deep in the book, left or right page, and where on the page to look, so I can usually find a needed reference within a half dozen page turns.
With e-readers, this is fuzzier. The "where in the book" depends on the progress bar, and there's no left or right to halve the search, so takes at least 10 - 20 page turns instead. These page turns are SLOOOOOW.
On the iPad, page turns are many times faster. So, finding reference material in a thick reference book that I cognitively mapped on the iPad is commensurately faster.