Symfony is a server-side MVC framework and solves many of the same domain problems as Backbone.js, etc. In this sense, no Javascript-based MVC infrastructure integrates better or worse with Symfony.
From the point of view of MVC, it is best to focus on the server-side solution (PHP / Symfony) or the client side (Javascript / various) and use the other to extend / support this solution. Although you could certainly mix these two, I think that if you start, your time is better spent using one MVC structure instead of juggling two.
One good thing about Symfony is that it can create Coffeescript. So neat, but it's just on the sidelines. Another thing is that Node.js allows you to write server and client code in Javascript, so stay tuned for more integration there.
Symfony MVC-, , , Javascript, DOM , jQuery Prototype, Symfony CRUD.