If you're considering E-Ink for environmental/ethical reasons, keep in mind that electricity may eventually come from renewable resources, while materials that go into electronics like E-Ink devices and tablets are definitely finite (and people die every day in Africa because of that). If you need to reduce eye strain and headaches because you read a lot, that's another story (blue light filtering glasses help, BTW). But if you already have a decent tablet, you're not doing the planet or anyone else a favor when you also buy an E-Ink device.

Edit: subaquamille raises great points about battery life and obsolescence which strengthen the argument for E-Ink.