I've been using CoffeScript and backbone.js a lot lately, it really has changed my approach to web apps (keep the server light, just transfer json back and forth). The approach that has worked the best for me is to keep pages such as the landing and authentication as traditional rendered pages and save the client side rendering for the user-authenticated 'meaty' parts of the app.
If you want to look a couple really-small examples I have a couple apps on github I used for learning backbone.js/coffeescript. They both use the underscore.js template engine.
I like the idea of doing authentication and OpenId in a traditional web stack, while doing the "app" portion in a javascript backbonejs stack.
That lets you use existing libraries (Devise, etc.) for the authentication and once that's out of the way, you can go have "fun" with coffeescript and mustache. :)
If you want to look a couple really-small examples I have a couple apps on github I used for learning backbone.js/coffeescript. They both use the underscore.js template engine.
https://github.com/jwarzech/realtime_hn Backend is sinatra, takes a HN story url and polls the comments in reverse chronological order.
https://github.com/jwarzech/inspire_board Backend is rails, polls images from dribbble and lets you save your favorites.