There is a nice run down/comparison of various MVx JS frameworks here
http://codebrief.com/2012/01/the-top-10-javascript-mvc-frameworks-reviewed/
it's followed by a good discussion in the comments too. I think I've seen Gordon (who wrote it) on here so maybe you'll get a reply from him.
I'd say if you are looking to learn this style of application development then on the one hand, the wealth of open source backbone examples around could be good for you. But on the other hand, although new, the Ember package is actually more complete IMO than backbone.
Both give you the ability to implement things in a variety of ways which can be confusing, but Ember provides more of the code that you would have to write yourself in backbone as standard which for me personally is more important for rapid prototyping than the wealth of backbone examples available.
There are more mature plugings for data persistence for backbone, but there is a great community buzz around Ember and lots of contrib libraries are making great progress. I've been pleasantly surprised with how quick I (and others) have had quality responses for a relatively new framework here on Stack Overflow.
When you say meteor we are talking about totally other stuff. If you wanted to do more with code re-use on both client and server side then take a look the recently open sourced yahoo mojito https://github.com/yahoo/mojito running on node.js - I've been messing around with it over the last week, and If you become familiar with backbone/ember or any of the others its a snap to get to grips with.
I should also say I have Knockout.js in use on some production systems but have never used it for a fully fledged 'application'. I've got it hooked up to a mongo interface for drilling down into and pageing logs stored in mongo. I really like it, but I wouldn't feel comfortable building anything too big in it.
Well, that's a bit of a ramble - isn't it :)
Quite happy to talk more about it with anyone who is interested. I've used a number of these frameworks in anger/production (including things not listed in the 'top 10' article) and people at work are sick of hearing me talk about JS i think :-D
