I saw this Mashable post recently: Kindle and iPad Books Take Longer to Read than Print [STUDY]. Jakob Nielson conducted a study that found reading speed decreases for electronic devices when compared to print. That is, it takes longer to read something on a device - in this case, either a Kindle or and iPad - than it does a book, anywhere from 6 to 10 percent longer. However, user satisfaction with those devices when compared to a book is about the same.
Personally, I have a harder time reading on a screen, which is one reason why I'm not anxious to get an e-reader. But maybe that's just my age and the time I grew up in, or maybe it's my fondness for physical pages. What about you? Do you have a preference of one format over another? Please feel free to explain in the comments.