Works well: can set default quality for YouTube videos, and this applies to embedded videos (YouTube videos outside of YouTube, on other sites). Can also set default playback speed, audio-language and subtitles on/off.

Only thing it lacks some similar extensions offer is an option to cap videos to 30 FPS, which can be useful on very-old PCs which may struggle with 60 FPS at high quality settings and when sped-up.