I was under the impression that this was more like ExtJS (i.e. the heavier end of the scale) rather than something like jQuery that pretty much just works on top of standard HTML markup.

I spent several months attempting to hack our applications to work within the ExtJS way, but found it pretty hostile to working with a traditional web application (e.g. POST/GET rather than AJAX everything). This bad experience led me to dismiss Cappuccino as being inappropriate for our needs, though it might be useful for writing apps from scratch. jQuery has proven far more amenable to a baby-steps conversion from a traditional CGI app to an AJAX application. There's also the accessibility and downgrade path question with all-JavaScript generated pages (like ExtJS, and, I thought, Cappuccino).

But perhaps I'm mistaken about Cappuccino.