The UI was super confusing to me because I've never seen an extension that is active in the address bar like that. Functionality is pretty limited, but it does what it's supposed to do.

It's better than nothing. It does only one thing: add articles to your account's default reading list with a single click; you can not view or edit the contents of any reading list via desktop browser, however, since I typically want to save articles while I'm on desktop to read on my phone later when I get a spare moment, and not vice versa, this is adequate for my purposes.

Good, works for me, but a even a bit more than it should – I could put on the reading list even my Wikibooks watchlist. (The app couldn’t open it, of course, although it had no problem with the Wikibooks main page, except for that it wanted to open every internal link in the browser.) Also, a notice would be good which would show if a page is on the reading list.