It will be cool if Meteor could be packaged in a gem and be embedded in parts of a Rails app where client side interaction is heavy.
Isn't Meteor written in Node? Maybe this is an ignorant question -- but how would you integrate it into a Rails app?
I'm not sure about Ember, as I believe it tends to be more opinionated about how much of the page it controls, but Knockout and Backbone are generally pretty easy to use for just a portion of the page.
Steve Sanderson (creator of Knockout) has a really good overview of the most popular client side options (both libraries and frameworks).
http://blog.stevensanderson.com/2012/08/01/rich-javascript-a...