We've developed (and keep developing) a rather big application in Backbone.js - it holds up and works perfectly.
We are setting a move from it since it lacks in some areas (no POJOs, no nested, manual binding, lots of extra backbone-mini-libs required), but it does hold well at any scale.
I wouldn't direct new comers to backbone.js anymore, new trends with 10x more contributors wins over anything. I wrote about it being the C++ of JS MVC frameworks (http://berzniz.com/post/66372634868/backbonejs-is-the-c-of-j...)