The comments above are entirely missing the key benefit of the current generation of eInk reading devices - their amazing battery life.
I know that I could use a tablet PC to read books on, but I don't want to be reading my books on a device that I've got to worry about recharging every 3h. I want to read on a device where I only have to think about recharging every couple of weeks.
OLED screens and all the rest of it are "power hogs" just like LCD screens are. They aren't a replacement for eInk.