While I agree that people vary greatly in their vision and what they are sensitive, too, and that is hard for those of us on either side of the spectrum to understand what others experience, I still have the sneaking suspicion that those who adamantly insist that they CANNOT read on a back-lit device have not explore the ebook reading options and display options that the iPad and its apps provide.
Sebastian, great work posting those photos of how dark an iPad screen can get. Your point about making the font size bigger is also really important. As is, changing the font type and colors of text and page--and the margins (just try Stanza or eReader).
All those require making some tweaks to the out of the box, default settings of the iPad, but they are easy to do.
I think some folks look at an iPad in a store, where they are turned up to full brightness, the text might be smaller, and decide they couldn't tolerate it.
Similarly, the sweeping declarations that e-ink devices are easier on the eyes are just that-- sweeping! For many of us, the darn flashing on page turns, along with the ghosting, is an absolute deal breaker.
Those who reject the iPad are missing out, as they could have one device for reading, music, playing, astronomy, newspapers, etc. and have a very rich experience, not just a single dedicated device to only ebooks.
Nevertheless, having options is good!