I've been evaluating Backbone, Spine, JavascriptMVC, Knockout.js, and Sproutcore 2. What I like most about SC2 is the direct support for key-value coding, including bindings and reusable well-defined controllers.

I also find the 5-layer [Model, Model-Controller, Controller, View-Controller, View] better for my OO coding style and isolation of dependencies — but that's probably more of a personal quirk.