I've been reading several hours at a time on my iPad with "retina" display, and the experience has been fine. I think the crispness of the text makes a huge difference. I haven't read for five hours at a time in sunlight, certainly, but I've read briefly in sunlight and it seems fine.
Then again, I'm fine with staring at my computer monitor for hours, and plenty of my friends can't stand reading on computer monitors, so maybe I'm weird.
One advantage of the backlit screen is that I can read for hours when it's dark (with white-on-black text) and it's quite fine.
(Also, since this is LWN, mumble mumble iPad mumble non-free software mumble locked-down bootloader mumble.)