E-books. By and large, nay.
The last couple I read through, as opposed to having around for occasional reference, I printed out and put them in a 3 ring binder to read from.
For one thing, I tend to heavily annotate some of what I read. Can't do that with an ebook.
For another, I don't own anything that would be real comfortable to read from like that. Laptop's too big, and requires me to cart along a small suitcase full of associated bits -- charger, mouse, usb cables, etc. Get me a netbook class unit, with a looong battery life, and I might change my mind.
Reading off a computer screen tempts you to start web surfing, with is absolutely antithetical to the sort of sustained linear concentration you need to read and digest a book.
I do have a fair collection of accumulated stuff.