If you mean device for e-book, I haven’t ever bought e-readers. Because I can read e-book with pc or mobile phone.
I think eyes’ comfort is the most important. Because we use it for long time to read. If eyes gotten bad once, they would not be recovered.
I don’t think that e-readers will replace paper book. Because there are many people who love the feel of paper. Moreover, people can write or underline something on the book with pens. Some people like the analogic sensibility instead of the electronic cold though it is uncovient.

I have never purchased an e-reader, if that is what you mean. I can use my computer or smartphone to read an e-book. Most importantly, in my opinion, is comfort for the eyes. We've been reading with it for a very long time. Once eyesight was damaged, it would not heal. E-readers won't, in my opinion, take the place of printed books because a lot of people adore the way paper feels. Additionally, pens can be used to write or highlight text in the book.