I have used SproutCore in the past. I was assigned a project to develop automated web testing scripts using the tool called Selenium RC. Selenium RC was built to target regular HTML id's and classes, but SproutCore compiles the element IDs so that the element IDs are pseudo random, so I had to figure out the API for SproutCore so I could fish out the element IDs from the view tree.
SproutCore has a close analogy to compilers. If you have too many elements that you're importing creating for your web page, there's a chance you'll have a namespace collision on the IDs if you were to build your application with jQuery. When you're building your web page with jQuery, all the IDs of HTML elements are globals. There's no such thing as local scope as in a compiled or interpreted language.
The last issue. SproutCore can be slow. But it's a price to pay