I'm implementing an API using Elixir + Phoenix right now and the community around this has been incredibly helpful. Despite the relative newness of Elixir, the ecosystem is really quite rich, at least partly because it can draw upon the entire Erlang ecosystem. Highly recommended!

Phoenix creator here. This is a huge release for us that really streamlines the development process and adds some big ticket items on our way to 1.0 later this year. I would be happy to answer any questions.