Personally, I found reader mode missed just enough — like images loaded with js or footnotes referenced in an article — that it was more trouble than it was worth. And as much as I liked the "readability", I missed missed seeing the different designs, fonts and so on. (Who would want to read all books with exactly the same layout and typeface‽)

Now I have a collection of user stylesheets I use with Stylus to improve sites I read a lot. I especially often remove fixed toolbars and adjust font size and line height. I also use the browser zoom feature a lot to get one-off sites to a better reading text size.