I print up all the ones I buy, but I only get adventures for the most part. The only thing that is slightly annoying is that the maps don't line up on both sides very well.
I like to have them to reference on my laptop. The most handy thing is a PDF that I can do a search on. Very cool for finding things quickly during gameplay or heck, even during planning.
Also, I like to be able to print out small bits to hand out or place in a binder.
And as I said in another post I would pay a little extra to have all the printed books I have purchased (which is quite a few) in PDF (good quality) format. I would like to be able to search and see everything clearly. So yes, this means I have downloaded a bunch off the net but it's cool because I own everything I still have on my HD (the crap gets deleted).
Actually, I have downloaded a few that I didn't own just to check them out and have made a few purchases based of my ability to sit down and look through it. Quinitessential Fighter is one off the top of my head. I also bought Spycraft based on the free preview that AEG made available.
I am also one of those people who likes to own the book so if it is available in printed and bound form I'll buy that as well. I have actually had to cut down on buying RPG books lately, I was buying too many.