pretty well actually. not much hype or talks about it anymore, but it's still used to get the job done.
In the world of Hype, I still use Ember at the frontend and Rails API at the backend.
Why use js in the backend if you don't have to? It's hard to read, maintain, and debug.
react-rails is really good.
react_on_rails is even better.
Not really. Depends completely on what you need. Not integrating with your existing asset pipeline isn't necessarily a plus depending on what your goals are.
There's also a weird amount of self promotion going on with that repo. Not sure what it is about that but it's kind of been a turn off to me.
Its not so different from thoughtbot's repos, just a different personality.