In order to make a reasonable choice between js MVC frameworks, the best is to write on your own a small app and decide for yourself. During my research, I have the following chart.
1. JavascriptMVC
2. AngularJS
3. Backbone
All the rest (Ember, Knockout, Dojo) aren't nearly close to the 3 winners.
Backbone and Angular were described quite good. So, I'll add what I've loved about JavascriptMVC (JMVC) and what problems it solves out of the box:
Problem 1: How to structure the project?
Where are 100+ variants to structure the folders of the project. It's important when is grows. JMVC has automatically generated empty project with very clean structure: separate folder with models, and separate folder per module for controllers and views.
Problem 2: AMD
AMD is built in into JMVC
Problem 3: jQuery support
JMVC is built on jQuery, so no collision happens what so ever
Problem 4: Documentation
JMVC has plenty of ot including API description, tutorials and active community
Additional sugar:
It has built it google closure compiler and minifier and checks your js code and optimize it;
It has built in documentation script that generates documentation out of comments.
Thus, JMVC is not just an Javascript MVC framework, it's great set of tool for the development of serious heavy apps.