For me, metaprogramming is one of the things that was a deciding factor with Elixir, in addition to all the other functional programming aspects that are part of the language. I'm most definitely not an amazing polyglot with a wide degree of experience with every single functional language out there, but this feature pushed me to invest time to learn Elixir. Metaprogramming has saved me immense time in numerous applications in Ruby. So, I consider it a critical thing to learn.
Of course, after investing time in Elixir you very quickly learn the power of the Erlang VM. Elixir is like a gateway drug! I now have a consistent mix of Erlang and Elixir syntax in the stuff I'm playing around with. One of the chief complaints for people with Erlang is the syntax. Elixir makes the entire proposition of learning Erlang far more appealing...and then you really it is all actually Erlang under the hood!
And, one of the big things that Elixir adds on top of the Erlang VM is metaprogramming. There aren't a lot of great sources out there on the subject. Chris's book is a great addition to the slowly growing Elixir library.