Probably not ready to be building 100 million user services on just yet, but utterly fantastic for quick prototyping & iterating services (which is a large part of what I do day-to-day).
I love Backbone & Ember and all the rest, but as mainly-a-designer/front-end-dev, if I can get away without writing an API in Rails and have everything just work I'll choose that any day.
I've also got this idea of a scale of 'magicness' from 0-10.
- 0 - writing the JS by hand, maybe with jQuery.ajax etc.
- 2 - Backbone - easy to use, easy to debug — just not very magical!
- 5 - Ember & Angular - pretty cool but still enough that the headaches can be off-putting
- 9 - Meteor - always seems to work, never frustrating, so magic that the occasional thing that's tricky to implement is totally worth the rewards.
edit: also, shameless plug - if you're in London and like tech meetups that aren't boring, come to the Meteor meetup. It's fun.