Yes and no - it depends™…
First the yes: As @RyanHirsch pointed out, the notice only applies to Ember-Data which is getting out of Alpha pretty soon if I am not mistaken…
And Ember.js itself is solid. I did a little internal app just recently and it worked like a charm for me.
Now the no reasons:
Ember.js is very young - not in the technical sense that there are bugs (there might very well be - but I didn’t hit any so far), but in the very real “I can Google my way to a solution sense”
Ember and it’s inner workings and APIs have been under a lot of flux while under development, and although public attention is good for a new project like this, in this case it also led to a ton of outdated or just plain wrong code examples, tutorials and blog posts on the web.
Things that may have been applicable to rc3 don’t work the same way in rc5 and a lot of times authors did not include what version of the framework they wrote the article against.
It’s confusing as hell, especially since Ember.js is quite a bit of Architecture and Framework to wrap your head around - but it’s very worth it.
Just don’t expect the level of documentation/tutorials and videos you’ll get for something like Rails or some new .NET Technology.
Once you get a better understanding for the concepts you start to understand how things fit together and where you are supposed to fill in the blanks - but still then you’ll at times find yourself looking for guidance from running code like the excellent Discourse source or directly from the Ember.js code).
Do it - but be prepared for some challenges ahead.