IMPORTANT: Use old version!
This is a great extension with many useful features. There aren't any alternatives offering the same quality. Amazing work!
HOWEVER, due to some issues explained on the GitHub page, most of the functions are gone in the current version. Until the functionality is restored, you should use the latest "old" version for now, which is 0.2.5. You can find the version archive under the "More information" section of the store page.

Used to work fine... right now though, it just completly breaks YouTube for me. Not a single YouTube video wants to load while this add-on is active.

It does everything it advertises and blends in so well with youtubes UI. I can't wait till they add more features.

No feature of the add on work, it's complete useless

I loved this extension because of the wealth of features. Until the features return I have uninstalled and gone with a competing extension.

for pity till blacklist features comes back, like in Youtube Plus, this extension is no better than any of several others.

This has worked so well... but lately, they updated with new UI setting with most of the features gone. Stated that it's "coming soon". Which is kinda stupid to do. They need to roll back the version and put the old features back in. They should only replace them when it's ready, not "coming soon" tacked on. The blacklist channels feature is one of the biggest reason why I use it... and it's not ready in this latest version. Stupid thing to do.

EDIT 2020-02-20: LOL, Even with new YT layout UserScript "YouTube +" concurrently with UserScript "Youtube Hackz Magic Fixer...("Classic Youtube Layout")" works.
I do not understand why Iridum was created and why use it ...

old comment:
Does not work... Video can't play after install Iridium...
The most funny and the biggest irony is, that userscript the "YouTube +" (see github.com/ParticleCore/Particle) works well... (I'm using Tampermonkey.) Are You sure that the Iridium is former the YouTube +? It does not look like that...

Just waiting for "Reverse that stupid playlist" button. Thanks for Dark Theme and Disable Autoplay features.

Good extension that I have been using for years on Chrome.
However, the "Allow loudness normalization" feature does not work on Firefox. It is missing in the latest versions, and it does not work on version 0.2.5.
Really hope that this can be fixed.

Completely useless.

Thank you Particle for being amazing. I've been using Iridium for so long. I hope that my favorite feature, reversing playlists, comes back someday soon.

I don't have a stable income yet, but once I do I'll support Iridium. Thank you so very much for the amazing Add-on.