HN user
Sproutcore has come along way since 1.0, I totally agree. However, the code that I've seen in sproutcore apps just didn't seem as clean, simple and intuitive to me as in backbone apps. What really got me going with backbone is that I could so easily refactor my exist jquery code into a backbone view, and that the models talked to my existing server app (rails) with almost no effort.
The important difference I see in the approach used by backbone is it gets us away from having the server be in control of navigation and UI events. After the initial page load, I'm exchanging simple data as json with the server. It wasn't HTTP I was arguing with, it is having a server side framework managing the UI navigation.
The best way I've heard coffeescript described is as a better syntax for javascript. Though at first this may not seem like a big deal, in my experience it's been a huge deal in the effect it has on how I approach client side code. Now that I can write code I really love on the client, it's greatly lowered the barrier for me to do so.