Every so often I find it necessary to move things from one repo to another and preserve history along the way. I've made do with a janky shell script that recreates all the commits touching relevant files in the new repo, but this looks like a much less sketchy way to do it. It even sends changes back to the original repo if I understand the code right.
git-subtree is for incorporating an entire repo. To cherry-pick a single directory from another repo you probably need git-filter-branch to split that out into a standalone repo first, followed by git-subtree. Still, this is just two commands so I don’t see why a third party tool is warranted, especially since the third-party tool here seems to require a more complicated process (maybe it achieves more than what I think it achieves, I just don’t see it from the description, especially considering the problem it set out to solve seems to be exactly what I had in mind).
There's basically nothing you can't do with filter-branch and the variety of filters it can execute. Moving a subdirectory to the root is trivial with --subdirectory-filter, but even without that, you could do it with --index-filter or --tree-filter.
subtree is more transparent to users of the repo as it is basically pulling in a copy of the repository during the clone. submodules on the other hand are only storing the metadata pointer to the child repo and require users to manually clone. I have more information regarding subtree (and subtree merge strategy) on this answer: https://stackoverflow.com/a/33579069/2105636
