Would you mind saying what you find most useful?
I have an aging iPad and would very much like to replace it with an a-ink device because I don’t like staring at a light source.
For me, the most useful trait is that it gathers all my note in a "physical" gadget rather than being scattered on several laptops/smartphones/notebook.
I'm on the device 6 of 8 hours in a day, constantly taking notes of my meetings and conversations. Frequently I write sentences in the wrong order, and have to rearrange them for logical/linear understanding.
Sure, you can do that on a Slate or iPad, but neither feel or sound like you're writing on real paper.
It is just so satisfying to use.
It's like being able to carry an infinite number of infinitely long paper notebooks with you.
I used to get through about one paper notebook a month, and couldn't find notes I'd taken 3+ months ago. Now everything I've ever noted is available with 5-30 seconds of searching and paging about.
The fact that, if I've written a bunch of notes in the wrong place, I can cut and paste them elsewhere is something I use every day. Everything is so much more organised.
Also, and it's definitely NOT designed as an e-reader, but if you convert your ebooks to fit the screen, it's approximately the size of a hardback book page, which I find miles more comfortable to read from than my Kindle.