It has a lot of the things that you'd want a youtube enhancer addon to have. But I perticularly like the ability to block ads (which I do anyway with uBlock Origin, but now have turned it off for youtube). The blocking in itself isn't the remarkable thing, but the ability to whitelist your favorite channels that you want to support economically, that you want to support so you leave the ads on because their content is so good. I absolutely love that. And if you're in a extra sensitive mood you can just click Alt+Delete and it will block the ads anyway!

The choices of different (mostly dark) themes is great, I only found one that I'd ever use, Mint-Y-Dark to go along with that theme, but I'm mainly on windows on my main desktop and youtube watching computer, but my laptop happens to run Mint and Mint-Y-Dark is the theme I use.
I just use the regular Youtube Dark Theme.