As @leushino is so strongly recommending, I will stay with 12.16.

The email and RSS/Atom reader is fully divorced from the browser, bookmarks are gone, I can't switch between tabs with the mouse wheel (and yes, mouse gestures are important to some of us).

Features like user scripts and downloads are present across browsers, but have changed somewhat in ways I find disconcerting. The spell check seems to work well. I've not used Dragonfly much in the new version. (It looks like it has all the "stuff", but some things like autoselecting items from the browser just don't work right.)

There are a number of Chrome-isms that I dislike, such as the large number of process threads it spawns.