I’ve also found exenv to be the easiest and best tool for keeping track of Elixir versions. Note that the local command is per folder so you can keep versions for each project. I use master for some projects but have locked some to 1.6.
I’ve been using 1.6 branch for at least two months in my projects. I had some minor issues with deps compilation but the overall quality is very good. rc1 looks fine too.
I haven’t formatted my whole codebase as there are cases where the formatter decreases source readability and it’s not worth the huge diffs.
I think that the changelog should mention String.split by "" as a breaking change though. IMO it’s previous behavior was not a bug to begin with and the change did break my code.
I’ll second that you should give a version manager a try. Any of the Elixir specific version managers will do the trick, but I highly recommend asdf.
If you work with any other languages there is a good chance you need to manage those as well, asdf will do that just as well as it manages Elixir. On our team we manage Erlang, Elixir, Node, Python and Ruby versions with the same tool.