I try to avoid buying paper books now, in favor of e-books. I haven't started replacing any of my existing paperbacks with e-books yet... I don't need 2 of the same book, in any format. However, if I ever find myself in such a space bind that I must give up my paperbacks, I may re-purchase them as e-books where I can.
I would be willing to make an exception in the case of a good and reasonable-priced e-book collection... say, the entire run of Doc Savage novels for under $50 (Sold!). Or the Clarke short story collection if it comes available for under $20 (definitely for $10).