The one thing my Sony does badly is pdfs - so if that *might* be important, try a few out on a demo model.
I have a freebie running on my PC which converts .pdf formats to Kindle. I now go to meetings carrying only my Kindle (which of course has loads of novels on it), my official documents, and usually one piece of paper, the agenda, to doodle on. Unbelievable freedom from carrying 50 or more A4 sheets of paper.
All of the books on my Kindle are freebies from Gutenberg, Kipling, Jerome K. Jeroma, Haggard, Wells, Wodehouse, etc.
With hindsight, I would not have bought the Kindle but would have gone for an Android tablet from Maplin, Amazon, or Morgan. They are the same price, have backlighting and a touch screen. Still, the Kindle's battery life is amazing.