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ngl #EmberJS has probably the best experience out there when it comes to upgrading.
The path and tools to help you do that are so well thought out that when I used to work with it, we were never behind too much.
With flags, you could live in the future with half the app in the past.
Sometimes I miss that.
Don't even get me started on the codegen and how you basically have everything laid out for testing every bit of your app.
Tests are a first class citizen and that is the most overlooked part every time.
You gen a component and you get testfile generated with it 
I'm going to be so pumped next time I fire up an Ember project from scratch, because there's been lots of goodies coming when I last worked with it 