Yup! The previous version used JMVC but it was a poor fit. Backbone does juuuust enough to help you out then gets out of your way. Means you have to write a lot of your own architecture but that's pretty much my favorite thing to do ever (and I hated JMVC) so I gladly did so.
The site is way faster now, despite having more stuff going on, just because half the codebase is no longer trying to hack around the parts of JMVC we didn't need.
Hey cowpewter -- I'd love to have you write up your experience for http://backbonejs.org/#examples ... email me a paragraph if you're interested.
Edit: One tip for you as well. I noticed that you're using "#!" in your URL routes. You probably don't want to be doing that these days -- either just plain "#" works, or pushState with hybrid server-side renders if you want to be indexed by search engines.
+1. I was using marionette, but again, it's buying into an entire system. I ended up writing my own lightweight (15 lines?) version of their region manager and use that now.
The site is way faster now, despite having more stuff going on, just because half the codebase is no longer trying to hack around the parts of JMVC we didn't need.