I'm not married to e-ink but for me it has the best reading properties I've seen to date.
I have a theory that the eye strain is more related to the optical noise then back light. The light source has a big factor in this but it's also the ongoing screen refresh. If you've seen a photograph of a LCD you know what I mean. My theory is that our conscience mind filters this out so we don't notice it but our optic nerves are working hard because of it. Some people are just more sensitive to it.
The ability to read in sunlight is also important to me.