I am an avid reader who just received my first Kindle as a Christmas present. A couple of books later, I have noticed that my reading speed on the Kindle is a fraction of what it is for the same text on paper. I decided to re-read a novel before starting its sequel, and after a few chapters switched back to my old paperback. I found myself reading about 3 times faster on the physical book.
This is at least partly because there is so little text per page on the Kindle, a function of both display size and line- and letter-spacing. But beyond that there is also an issue with readability or legibility. The font used on these devices is remarkably ugly.
These are not permanent, unchangeable characteristics of E-readers, so I hope things will get better as the products evolve.