It ACTUALLY reverts the ugly new video UI back to the way it used to be. That is literally all I needed this for. Good GOSH, I'm getting so sick of having to install a new extension every time YouTube does something...

That being said, I'm also very grateful it's an option available to me, or I probably wouldn't even be on the internet at all any more. Thank you, extension developers, your work is appreciated.

EDIT: Forgot to mention that, for some reason, I have to refresh the video page (sometimes more than once) to get the old UI to apply. Not sure why, but it's worth it, and still better than my previous method of "embed the video into a Tumblr post and just watch it in the 'new post' creator".

EDIT 2.0: It seems like the UI "sticks" when you don't open a new page. Like, say, you have to refresh the page a few times, and finally the extension kicks in, and fixes the UI. Opening a video in a new tab makes the new UI come back, and you have to refresh again, and it's annoying. But if you're in, for example, a playlist, you get to the next video, and then the next, and you're five videos in and hey! You haven't had any trouble with the UI reverting. Nice!

TL;DR: Refresh the page a few times to make the new UI go away, then stay on that page by using a playlist (copy+pasting URLs into the tab doesn't seem to work). I use my Watch Later, because I've already used it as long as I can remember, and it's convenient to just hit the little clock icon and have everything ready to go.