I have a Tablet-PC (ThinkPad X41). I bought it, because the idea of writing on a digital display instead of paper was compelling to me. (easy deleting, no paper mess)
Let me tell you - writing on a Tablet-PC sucks!
You have a much worse feeling, what you are doing, than on paper.
(this is of course my subjective opinion - but I'm usually not very picky... - if it were at least as half as usable as I hoped it would be, I wouldn't write this)
And just you know - reading an e-book for a longer time on it, makes your eyes hurt...
But it's a good question, I'm also wondering, if writing on a eReader is something, that owners do.