Ember has a little bit of a learning curve, but once you master that - web apps are so simple to build. With plugins for Rails controller/route generation - you can forget about needing to write complex server side code and focus mainly on the core in-browser app. The data-binding and computed properties make development in Ember so much fun.

The best about Ember is Convention over Configuration in its tool - Ember CLI. The organisation and structure of complex projects is great. Also, the components, the bindings, Ember Data, routing, everything together gives great experience of creating complex SPA applications. Ember Data makes it straightforward to work with your API backend and manage models. Compared to for example Angular I'm happy that Ember comes with Ember CLI. You have best way how to do things shown in the Ember Guides. You don't have to find how to organise project in some tutorial or article like its in Angular, but you can actually jump right to developing applications by running simple command ember new my-new-application.

Everything. We love Ember.js! It is extremely powerful. Their documentation is awesome. The framework is being updated at a high velocity. They have released stable builds in a timely manner. New features are being added frequently. We use this framework for building powerful web applications.