Magit[0] is so good that I haven't felt any real need to use jj... yet. I'm sure I'll switch if it gets emacs integration of a similar level to magit, but the one I tried[1] isn't quite there yet.
I think the big thing (potentially, for me) is the ability to postpone conflict resolution during a rebase. That can be quite painful in regular old git, but git-mediate helps make that less painful in practice in my particular situation and workflow.
We'll see once better non-cli UX appears. I'm low-key excited for what could be possible in this space.
I am excited too! It is probably too much to hope, but I nonetheless am hoping that magit gets a jj backend before I have enough motivation or need to learn a new tool to do the same old stuff :D
[0] Well, plus git-mediate for solving conflicts
[1] https://github.com/bolivier/jj-mode.el