Well, I tried Ember back in the day for one project and remember it being pretty hard to learn and quite fat in terms of file size. It was quite powerful when you were using it the way it was supposed to be used, but made your life pretty hard when you wanted to do things a bit simpler and the initial load times were horrible at the time.

I heard, that things got a lot better over the years, but I never tried it again. However, I still admire the very clear architecture they had with ember-data as it was very useful (e.g. for testing) and eliminated a lot of uncertainty regarding the API design.