An iPad (and I assume the other tablets) make the PDFs more usable. I fired up an internal web server on my machine and put some pdfs on it, then browsed it with the iPads web browser. When you do that, iBooks will ask if you want to add the PDF to it. When you say Yes, it downloads it to the iBooks reader on the iPad. Then you can bookmark and arrange the books, etc.
Granted, something like an iPad is an extravagance (and not exactly a cheap one), but it does make working with electronic books more friendly, and they will only be getting cheaper as time goes on.
For example, I once bought an obsolete Palm Pilot solely for use as an e-book reader off of eBay for, like, $20. It was perfect for reading just text books (like Baen Free Library). The small form factor isn't very good for PDFs though.
But the large devices work pretty well.