I got a Dasung 13" one a while ago, the larger Dasung wasn't launched yet (and I was struggling a bit to justify the price tag of the 13", now I'd probably go for the larger one). It's very limited, basically only works for black and white text, but for that it's great. I use it pretty much daily, just keeping a shell open there. I've changed my terminal to be black on white and all colors map to light/dark gray (for light/dark versions of the basic terminal colors), which works decently. I did change themes for things like tmux and vim to work better on a grayscale screen.
Reading text on it is great, but compared to a Kindle Oasis (whatever the newest model was last summer) its DPI is quite low, so you can't have text as small without it getting blurry. I mostly got it because I noticed that I was getting some eye strain staring at a regular screen all day and as best as I can tell it really has helped with that. It's also powered off of USB, which makes it usable on the go as a second screen.
My main gripe is using a Mini-HDMI-Port with a custom cable to inject power from USB (I think HDMI->Mini-HDMI plus a separate USB cable for Micro-USB input also work, never tried), but it's also a bit annoying that the inputs are on the sides and not the back. I also use it with a monitor arm at home, the included pole for standing it up is ok for on the go, but I really wouldn't use it permanently.
TL;DR: expensive, but if you want to reduce eye strain while reading a lot of text it really helps. Main driver for shell / coding tasks and I sometimes read longer websites/Google Docs/etc on it, though less frequently.