I really wanted to like Ember.js, but I had difficulties writing tests for my app and there isn't substantial documentation for it, which was a deal breaker for me.
We're using emberjs right now as well. But the lack of test integration is a huge problem for us. We right some tests with busterjs currently, but none of these tests are unit tests and tell us what exactly went wrong — somewhere.
So not to mis-read my comment: our application works and we deployed to production, and it's being used by customers, etc.. But whenver we need to dive into an emberjs-specific issue (e.g. we had a few regressions to fix which were introduced with the RCs) it gets messy.
If anything, the lack of testing (and currently lack of ideas how to integrate any) is the number one reason why I don't want to use emberjs again any time soon.
ember-data seems fine for us. Lots of conventions to adhere to (otherwise it's no fun at all). Once you figure out the side-loading, you're fine. We had to write a custom adapter so we don't have to repeat the things they call REST on the server.
A big fraction of the Ember community uses ember-rails, and tests using standard Rails tools.
For those of us not using Rails, here's a jsFiddle that shows how to test an Ember.js application: http://jsfiddle.net/ekidd/hCsws/
This includes both unit tests (for models, views and controllers) and full integration tests (using jQuery and chai-jquery to drive the GUI). There are one or two rough edges, which are documented, and which I plan to submit bugs about soon.
