Author here. I haven't done too much with Elixir yet, other than this Phoenix app, but I am totally enamored of it from what I've seen.
First and foremost, the community is absolutely the friendliest, most helpful group of folks.
I'm happy with its performance. Phoenix feels similar to rails to me (I'm primarily a rails dev), but with easily 10X performance. And the concurrency. Oh, the concurrency. This is Erlang's bread and butter, and Elixir and Phoenix are built to take complete advantage of this. I love that every request gets its own Erlang process, and you don't have to worry about it blocking anything else. The Erlang scheduler will keep it fair, cutting off any long running computation if it has to, to keep the latency down on new requests.
I really like how interesting and mind bending Elixir's purely functional, process-oriented approach is. Nothing (well, very little) is mutable! You can sort of simulate mutation by spinning up a little infinitely looping tail-recursive process and sending messages to it. I encourage you to go through the excellent Elixir Getting Started guide [0], and in particular the "Processes" section for more on this.
But I think the thing I like most about it so far, is the introduction to battle-tested Erlang approaches to robustness (OTP). This is a set of abstractions and libraries that have been iterated on over the years and are a fantastic way of building an app that's resilient to failure. Elixir, as it does, takes these abstractions and libraries, and puts enough sugar and consistency over them to make them a joy to use. I find this approach [1] supremely elegant (it's erlang, but applicable to elixir).
Elixir is amazing. It's homiconic, like LISP, so being able to execute code at compile time can have a game-changing impact on reducing boilerplate.
And in addiction to that, it's functional, so I can get the awesome benefits of immutable programming, but without the dizzying complexity of, for example, Scala's type system. And it's based on the Erlang VM, so I get the benefits of battle-hardened concurrency and IO underpinnings.
Previously I stayed away from things like Go and Node/Express, because I felt like it wasn't as easy for the type of work I do as Django, and was missing key things like the admin CRUD interface. But when I looked at what's in the Phoenix framework[1] and read the source, it started to feel like I'm witnessing the dawn of the next Ruby on Rails revolution.
http://www.phoenixframework.org/
The Phoenix framework is really well thought out that making CRUD applications is easy and has all the bells and whistles you'd expect from a framework like Rails. But obviously that's just the surface. You have access to OTP with erlang's processes, supervisors, ETS, etc. You have access to every erlang module that's been written. You also have access to Elixir modules that load in to a Mix project like ruby gems. Also, it's wicked fast and scales.
But maybe best is the documentation is great. From elixir's docs to Phoenix to code docs, everything is clear. The community is quick to respond on IRC or GitHub.
For downsides, just newness: the lack of good blog posts make me feel like I'm re-inventing the wheel sometimes. Additionally, there's not many questions up on StackOverflow so my errors often come up with 0 google hits.
