I finally got around to testing Alexandrite, and personally I prefer it to either the default or Photon.

1. I love the ease of customizing the UI, especially the colors! I was able to dial it in to make it incredibly easy on my eyes, where as I found Photon's dark mode too dark, and certain elements needed more contrast. This is a killer feature for me.
2. I really like the dual-pane and overlay post feature so I can read posts and navigate around without having to load a new page, very nice.

I'll be switching to this as my default, I hope it survives the great UI Race 😄

Of the three (default, Photon, Alexandria), Photon is my favourite by a long way.

I use dark mode on everything so I have that bias. I find Photon easier to read than the others site to the colors.

The larger image preview is easier to see. I prefer text-only in general, but I notice my eyes are less strained trying to make out thumbnail details with Photon.

The UI layout seems tidier too.

Is the long term plan to keep them optional or phase out some in favour of others?

It's so nice to have a choice between the different interfaces!

For me I kinda like photon, but Alexandrite looks also pretty neat, especially on smaller screens

I like the standard UI. However, if it is inevitable that the site switches, I do prefer this Alexandrite UI. The Photon UI seems much more clunky. And my ADHD has trouble focusing. I do notice that it does allow you to easily look at users' post history, which is good for moderation, but Alexandrite feels like it has more user related features. I even tried to go into Photon's settings and tweaking the front page. Nothing I did felt really as comfortable as the Alexandrite UI. My vote is here.

I like this UI better because it's more compact than Photon, but I like both so far because it's easy to turn on dark mode. I also like the previews better than old.slrpnk. I like how Alexandrite discussions open up in something that lays over the rest of the content, and I could close the sidebar to get a better view. I'd like it better if I didn't have to scroll back up to the top to close the discussion, but maybe that's a work in progress or something I could post as a wish on their development site.

PS—Oh, I can just press Esc. Oh, I think there's also a navigation bar that will let me close the thread.

Also, I like how it shows the number of unread posts since my last visit, but I don't think there's a way to either take me to these posts or to highlight them while I'm scrolling the thread.

The search function appears to be broken/bugged on Alexandrite at the moment. Any searches will eventually result in an 'Internal error', with the reload button doing nothing. Search on the default UI as well as Photon still works without issue.

I also noticed just now that there doesn't seem to be a way to add screenshots or pictures via a button in the Alexandrite comment editor, had to switch back to the default interface to add it. EDIT: That feature is planned to be added.