I think the most important criteria with a reader (aside from hardware quality) is whether you're comfortable going outside the manufacturer's store to buy DRM-free books, or at least ones that can be liberated from DRM for future proofing. Calibre still speaks the format of these old Kindles, so they're usable, I expect that will continue to be the case for Kindles. If format conversion is too annoying to deal with then it's better to read on a general purpose iOS or Android tablet. I have a Boox NA4C and it's ok, nice hardware, but I have noticed the constant phoning home and am annoyed by the GPL issues (not that I expected a Chinese Android device maker to be fulfilling their open source obligations). For that reason and others I've mostly come around to just reading on a phone and tablet with non-eink screens.