I completely rewrote the README recently to be more "user friendly"; does it address some of your question? I'm not trying to be snarky, I'm genuinely interested in if the README is now tantalizing enough to make you interested:
https://github.com/martinvonz/jj?tab=readme-ov-file#introduc...But in short, it has a better UX than Git by a mile while remaining interoperable at the storage level, so you can use GitHub; it has many of the niceties of Mercurial's UX like revsets, no staging area, and a templating language for log output. Conflict resolution and rebasing is clean, easy, and nearly automatic. It has features neither support, like a real undo command and "operation log"[1]. The UX is smaller and conceptually cleaner; fewer "verbs" that operate on less "nouns", so many things are more regular. It is available as an API (Rust crates) so you can extend it naturally to handle your own workflow, without monkey-patching things. You could even use Mercurial itself as a storage backend, or a custom, centralized server like Google does.
Most importantly though, its internal "automatic snapshot model" design is quite elegant and makes the design and internals very clear. And storage system is completely independent and abstracted from this, and many of the other algorithmic details (Git sort of intertwines the data model, the algorithms, and the UX in various ways.) Features like hg changeset evolution, git rerere, and rebase --update-refs are all obsoleted and naturally handled by this design, among other things.
Martin, the lead developer, also had a talk at Git Merge 2022 that covers many of the fundamentals. They're all still the same but we're evolving many things rapidly: https://www.youtube.com/watch?v=bx_LGilOuE4
[1] Yes I know about the reflog, no it is not equivalent! It only tracks changes made to visible references ("ref" "log") while jj's oplog actually tracks commands invoked, and lets you completely undo individual things in the blink of an eye, or rewind if you wish.