I only want PDFs of books which I can't get in hardcopy, which isn't really an option up there.
-O
This, mostly; but also material that I don't want/need in hardcopy format. First, I love that the .pdf market has opened new doors for 3rd party publishers, free stuff (samples and full products), extra portable copy of my library, etc. It is just an excellent choice for fast, simple distribution. Also, I liked having the option to pick up Out Of Print books from publishers (including WoTC). Much better option than wandering the secondary market for a wanted module, source book, magazine.
I've purchased many .pdfs for both of the above reasons. When I'm gaming, I mostly use the hardcopy versions (original printing or printed .pdf), but having the .pdf option is excellent for prep & for amassing a huge library of resources that don't need to collect dust.
I'll leave the cost analysis to the MBAs, but I know I've purchased a LOT more gaming material because of .pdfs than I ever would have if just printed materials were available. (And I've gone on to purchase many printed books because I liked the .pdf sample/copy/publisher so much.)