Works perfectly for me, does exactly what the description says (in all parts). I have no idea why it didn't work for other people.
Depending on your keyboard layout (or personal preferences), you may want to change the hotkeys in the extension's options dialogue, though.

It can even work in conjunction with Subadub to have two subtitles for certain films. I just wish that one or both of them were movable, so that multi-line subtitles wouldn't overlap. Subflix practically uses Netflix'es built-in subtitles, so I'd say that's up to Subadub.

Absolutely wonderful addon. It does what it promises and makes the experience of watching Netflix that much smoother. I love the subtitle history feature the most as it allows me to clarify whatever word I misheard with minimal distraction.