how close is git-subtree to svn-externals? I've always had people complain about moving from svn to git because there wasn't a feature like svn-externals. git-submodules obviously didn't cut it.
Subtrees are quite different from either. They make the imported repository literally a part of the commit graph of the parent tree through a subtree-merge-commit. It then does some extra work to help you bring in new changes or export new changes back.
I think it's a better approach than either, but it's also not exactly the same thing, basically.