ipad 1 experience:
I recently acquired an iPad1 for work. I have not been on the iPad "bandwagon" - with a macbook air and iPhone I have not felt a need for an iPad before. However, I have been in a class with voluminous documents. Those documents happen to be available via PDF. I borrowed the iPad 1 to test the utility of an iPad for PDF use in place of the exceedingly heavy 3-ring binder I was using.
I ended up using iAnnotate to read the PDFs. That meant I could annotate/mark up the PDFs. Ibooks does not let you annotate PDFs like it does normal ibooks. I used BOX to copy files from my computer to iAnnotate.
All-in-all I found it usable but a bit of a pain. It was not a home run experience, but did make it so that I did not have to travel with a thick 3-ring binder.
If I just want to read PDFs in ibooks, the iPad is a much better experience - not so much "fiddling around".
I find the iPad 1 to be perfectly usable for this use even as the "new" iPad retina is available.