I'll put in my 2 cents. I have one experimental project using AO3 bookmarks that is my recs for one fandom. It's about half external links and is very much a pain to make corrections to. I don't use the system at all for myself, and I won't until it will handle external links as easily as it does internal.

I know there's heaps of coding to do on the archive as a whole, and I hate to be a downer here, but I seriously don't think those few bugs are really going to make the bookmarks more attractive to users than they are now. They function okay as a list of AO3 fics for the user's benefit only, but they have little function beyond that as any kind of resource in the ways fandom used Old Delicious.

As a reader looking for fic on the archive, I don't even know how to go about using the bookmarks to find something to read, and I don't bother. I think a walled off tag system for bookmarks with its own tag cloud and filters would help, but right now, almost no one even uses tags when they make bookmarks and few people even make comments.

I do notice bookmark counts on stories, but really, they're kind of like extra-strength kudos from that perspective, not anything more meaningful.