I've become more and more of a pdf purchaser over the years. As others have mentioned, only a small subset of most products that I purchase ever gets used, so pdf is actually a pretty handy way of not cluttering up my bookshelf. Plus, since I can access my home computer from work, they're a lot easier to reference during the day than any of the print books that I have.
Now, since I produce pdfs, my interest in them has increased even more. I'd say that 90% of my RPG purchases are now in pdf form. Big rulebooks that I reference during play I still prefer in print form, but pretty much everything else I buy is electronic.