Pretty good, love the speed control. Would be great if you could set it to open music videos with 1.0 speed and other videos with the configured speed (maybe it could check the section video's in, music is usually in "Music" section).
Other than that, the biggest issue I have with it is that it doesn't seem to be free and open source.

Really, Really like it when it works. However the menu settings are over the top complex.
Big failure right now is when the video playback is increased followed with a Volume click, the playback is reset to normal and fails to work until the video is reset. This fail is really a trial, since most videos need the sound boosted and increased playback speed.

It has been a handy little tool when I want to expand the volume for speakers who are not loud enough and I like to speed up the video when needed. I need to explore this add-on more, but so far it has been great. Thanks and keep making it better.

様々な機能があり、重宝しています
ただ、ループ再生機能が一回しか効かなかったのと、
アンケートがスキップされなかったのが気になります。

I would love it if this extension got a sharpness filter added, was a bit disappointed it wasn't one of the filter options.
Not to mention an integer based scaling option for people with 4k displays when watching 1080p videos, though I don't know if that's even possible.
A sharpness filter would help a lot though, since Nvidia's edge enhancement doesn't work in Firefox.

Does a wonderful job in most cases but does not stop youtube from auto playing in background tabs neither in firefox, chrome or opera. I think Youtube is overriding your setting somehow being the spammy ad monster it is.

One other small issue:
Sometimes videos don't start out in my preferred quality. I set it at 480p for this silly laptop, for instance). So videos wont even start because they're trying to play at 1080 or 720p instead. I noticed the same issue in Opera.

What an absolutely fantastic extension. The key things I use this for are to block ads, stop autoplaying the next video and set the default playback speed to 1.5.

Love this addon, can't use YT without it! Would love to see an option to sync audio as on some videos the audio can be out of sync due to uploader error and it really bugs me! Other than that this is a MUST extension if you ever touch youtube.

Absolutely fantastic, I love this add-on! The only thing I'd like more from it is the option to disable scrolling down the page while fullscreen, otherwise this is perfect.