It's funny how much I find myself using it since I've switched full time to Firefox. Site has never been tested on wide monitors? font-size is tiny? Terrible color choices? Annoyed by the font? Don't like flashy sidebars next to the content? Reader mode has you covered.

Previously, when reading longer pieces, I'd go into dev tools and quickly kill the side bar, set a max width etc, but it's so convenient with reader mode.