I am taking it a bit on faith that this work will pay off, even though my usual development "use cases" are fairly simple web apps where I just use a little AJAX (often pjax). I am assuming that if I master Ember.js then I will build more complex stuff.
I like Ember.js and Node.js - a light weight development kit. A few years ago I was doing GWT/SmartGWT for a customer: produces nice interactive web apps but development feels like running through mud.