I don't know what it is about reading on my phone screen -- I can do it for ages if it's stuff that I'm just skimming -- news, blogs, forums, etc, but if it's a long-form text I just cannot concentrate on it, my mind starts to wander, and it just bugs me. I can't explain why, it just is. And it's something specific to how the newer phone screens work because I used to love reading long form on my monochrome Palm III with electroluminescent backlight too, especially with the reading apps that could auto-scroll and you could easily adjust the scroll rate using the arrow buttons on the front -- I read many long novels that way.
Compared to phone screens, eInk is a joy -- I can get just as engrossed as I used to on paper books in the old days, and I don't need to hide under the sheets with a flashlight anymore!
My first Kindle was great for reading indoors or out, and had a book-light built into the cover that you could just pull out of the corner at night. The modern ones are even better, with adjustable warm lighting, making them "just work" in any lighting situation. Unlike a lot of the previous posters, I don't find size to be the advantage at all, and actually wish for a 5" version of the Clara HD as I value portability (I carry it everywhere! -- well, used to... haha, hopefully will again soon!) and actually like the feeling of making progress by turning the pages more.
A lot of my work involves text on screen that isn't changing too fast. I hope for a day where I can get a decent sized eInk monitor to put next to my LCD monitor for working on source code and documentation.