I used it a month or two ago and found it unusably slow but recently gave it another shot after they released an update and now it's pretty much on par with Sublime on my system. It might be worth you giving it another go.
Cool! Though, I think it would work better if items were placed on mouse up, that way you could tell if the user was just trying to rotate the display not and place something.
The problem Backbone.js solves is primarily that of structure and organization. It's not about replacing jQuery, its about how you organize that code and have a clearer separation of interface, data, and logic.
The font colors on that page, especially in the code snippets are so low contrast that I can hardly read it. It's a topic I'm interested in but the styling is keeping me from reading it.
I know that there are bookmarklets, etc, which will make it readable, but the author should take note.