Personally I have gone all PDF in the last 2 years, though I still have all the books from before then.
As my job has become more digitalised, it has been of massive benefit, so that flowed into my gaming life. Besides, living in a rural area, its hard to get to a store and harder again to find gaming groups.
Well... not to wax biographical, but in the past five years, I have to agree it's been a godsend that much, if not all, of BT material has been converted to (or issued in) electronic format. I have a co-writer in Korea, another in Germany, still another in Australia and editors in the UK, Singapore and on a carrier in the Persian Gulf. So it was nice to be able to send them the pertinent reference materials over the internet as they needed them. And if you don't need it anymore, hit the delete key.
And now that I have a good laptop with a working battery, yeah, the business of getting to a rule can be a bit easier. But it's still a pain hauling the lappy to work for to have a read while smoking, or taking a 'break'.
For gaming groups, I have found the paper version a lot better. Easier to pass around. Sure, you could simulate dice on your laptop. You could even simulate multiple such dice, easing the firing of a couple of RAC-5s. But who would do it very often when they can carry their own dice?
So for me, rulebooks should be paper. Reference books in PDF? Fine.
Cent13