I joined the sapling/subversion company this year, but haven’t had the chance to use jj. But given its resemblance I must say sapling has been great. Much more intuitive than git, and I find commit stacks much easier to follow than branches. I do wonder how it will work without the level of support of Meta, since you won’t have the same commit stack review UI (basically a series of pull requests being reviewed at the same time). So something like what this author is working on is needed.