The problem with Coffeescript is if you already know JavaScript, it's completely unreadable. Looking at code like this, I have absolutely no idea what it does:
class ItemView extends Backbone.View
render: ->
@options.items.each(@renderItem)
renderItem: (item) =>
@el.append item.get("title")
It doesn't make sense because there are magic variables appearing out of nowhere - @options, @el, append -- what are these? I'd have to study the resulting JavaScript to understand what it's really going to do.