Great question, Bob. I'm very interested in seeing other people's answers.
For me, a PC until about 10 weeks ago. Now, I'm experimenting using my Sony Reader. For that, I have more or less successfully converted with PDFLRF from either original PDFs or Word documents that I first convert to PDFs.
Am I happy? Well, they are still pretty hard to read sometimes. Some of it is due to my lack of spending time to play with formatting options. So figures and tables get cut in half and I end up flipping between pages trying to see it. If I'm really lazy with formatting, yes, eyesight's an issue.
But maybe more difficult for me is my frequent need to flip between different portions of the document or even different documents. Even a perfectly formatted ebook won't help that. As an example, I've been trying to review a calculus ebook to help my daughter and I find it pretty frustrating to read a problem at the end of a chapter, look back in the text for the relevant equations, look elsewhere to review the examples, and also look at the back of the book for reference equations in the appendix (hehe, or for the answers!). It's too hard to do that on my Reader well even with well planned bookmarks and it's much too slow. Maybe it's asking too much in this case. It's much better on a PC (larger screen to see more and also it's easy to flip between multiple open windows). But it's really trivial with a paper book: a few sticky notes and you can go back to learning and stop fooling with the medium. Still, it's been fun to try. And you gotta love the incredible portability of it. Terrific for productivity - if it was just a bit more practical.