I'll also say that after spending some time with these frameworks and being sold (I especially liked Backbone.js) I gave jQuery another shot. I ported all of our common libraries to jQuery and I spent a few days getting used to it and learning how to do certain things. I couldn't stand it. The thing that got me most was that there is no distinction between a DOM node and a collection of DOM nodes. Maybe I'm just too used to Moo (4+ years).
So we decided to roll our own. Composer.js was born. It merges aspects of Spine and Backbone.js into a Mootools-based MVC framework. It's still in progress, but we're solidifying a lot of the API so developers won't have to worry about switching their code when v1 comes around.
Read the docs, give it a shot, and let us know if you have any problems or questions.
Also, yes, we blatantly ripped off Backbone.js in a lot of places. We're pretty open about it, and also pretty open about attributing everything we took. They did some really awesome things. We didn't necessarily want to do it differently more than we wanted a supported Mootools MVC framework that works like Backbone.