Let's see:
* I use it daily, but really only for my shell, including vim for coding/writing configs. Depends on the day, sometimes I spent more time in the shell than in the browser, sometimes it's mostly browser (and thus plain old screen, not E-Ink).
* I think that's hard to say since my main screen is so much larger. I did notice a difference in the evening between a tablet with Youtube vs a Kindle with a newspaper or a book. Although I do remember one or two evenings where I turned off the main screen and only used the E-Ink screen - I think there it helps with going to sleep later due to less blue light.
* I haven't noticed any degradation so far.
* I like it. I've barely had problems with eye strain anymore and eye strain was the main motivation for getting it. It's also just a bit larger than my laptop, so it's great when I travel somewhere where I'll use my laptop a lot since I'll get a second screen on the go/for the hotel room/etc.