I read a lot of books, and while I wouldn't mind an eReader on the side, it wouldn't substitute for the actual thing.
And I find it different than an mp3 player in the sense that consumption of music you bought digitally and music you ripped from a purchased CD is somewhat the same.
However, the physical nature of reading a book is very different from the process of reading a book on an electronic device.
The other thing is that I am a big reader, but I never saw the appeal of having a hundred books at my fingertips. I can understand the need to have your mp3 library at your fingertips while in the car, bus, subway, or anywhere away from home, but not a book.
When I am reading, I am typically in a comfortable place at home. If I find that I can't tear myself away from what I am currently reading, I carry my book with me where I go. Heck, it can be a nice conversation piece. I had the pleasure of meeting this hot chick back at Niagara college, who for some reason was also reading the same Raymond Fiest book as I was.
As I said, I can understand the appeal of wanting all your music available to you at all times. I can understand getting the craving for a certain song, while waiting for the movie to start at the theater or whatever... but I never found myself away from home and thinking... "Oh my god, I simply *must* at this very moment, read that part again where Snape raped Dumbledore, "