The Kobo Glo
I bought my Kobo Glo #eink e-reader from eBay on October 25, 2019 for $23.50 plus about $10 shipping (though it looks like costs have increased since), and I’ve been using it for about a year and a half. Despite a few issues, it’s served me pretty well overall, and I see myself continuing to use it for at least a few more years.
Portability and Size
My Glo has a bookshelf’s worth of books on it—not a whole lot, but certainly more than would be practical to move—for both leisure and school/career reading. I don’t have to worry about the physical space my books take up. At about 4.5 x 6.2 inches, the Glo fits snugly in my pocket or between books in my backpack.
Battery Life
In sleep mode, the device can last weeks. I haven't tested the device with constant use, but I almost never need to charge it. I do most of my reading at night with the frontlight enabled.
Operating System
The Kobo Glo does not run on Android, unlike newer E-Ink devices, but instead uses a custom Linux-based OS. This is a little tricky, and my experience tinkering with Linux systems is definitely what made the Glo usable to me at all. When I first opened up the device and charged it, it refused to boot into the home screen, demanding to be either connected to a computer via USB or connected to the Internet. Neither of these was an option, since the WiFi driver was broken and I had no micro-USB compatible with the Glo’s data transfer.
Ultimately, I needed to open up the back of the Glo, remove the microSD card with the operating system, and write an updated version to it (in this process, you can also replace or supplement the operating system with something like Plato or Koreader, or just bypass the Kobo sign-up step).
After trying many operating systems and document readers, I've settled on using the stock OS, Nickel, for now.
Update 08/31/2021: I've switched back to using Plato, since it's much faster than the current version of Nickel and allows me to annotate PDFs.