I mucked around with it once. I haven't found a project to really use it on yet, as most of my projects are more document oriented. However, in general, I feel as though a lot of javascript mv* are applied to too many situations. They really make sense in a web application, not a site that is primarily document based like wikipedia, reddit, etc...though the lines between "application" and "document" really get blurry for a lot of things. While it is a nice idea to have the server side really just act as an API, I've yet to see a client side templating system that is as friendly as something like jinja2. I think a lot of the javascript solutions mix a lot of the presentation and logic/behavior, which I'm not a fan of. Anyway, things like backbone.js and angular.js definitely have their place, but they seem to be a little bit of a thing right now, and perhaps extended to things they don't lend themselves that well to.
Oh good, I'm not the only js programmer here. I never understood backbone.js - it's not mvc, and the * part of mv* is so random in it, it's hard to figure out where to put logic sometimes. I'm still looking for a good structured mvc library/framework. I found something really cool called Meteor, but that's a little too structured and repetitive and synchronous, and I found socketstream, which is promising, but it isn't ready for gametime yet. I just program for fun, so I haven't made a huge app yet, but my stack consists of:
node.js for backend plus express.js
mongodb for database
as my apps haven't gotten that large yet, I haven't felt the need to use an mvc framework - yet - so clientside I use zepto.js plus vanilla js (I organize my files, but I don't have a defined mvc/mv* kind of setup)
I don't work with immense amounts of javascript but I'm slowly (ie: lazily) trying to learn more. The list of examples that utilize Backbone is impressive though. http://backbonejs.org/#examples