Pro tip: Stay as far away from YUI as you can. YUI is <i>painful</i> to work with. Even after the changes that were done in the YUI3.x releases, it's still a nightmare. Syntax is awful and all of the various components you have to configure together are just more levels of bloat than you need.<p>People use jQuery + <insert other js frameworks> because they work better, are easier to use, are lightweight, and don't trap you into using the entire monolithic package that is offered by YUI.
this is a link to a person saying i like YUI because it has a gallery and modules, oh and zillow uses it. case closed!<p>i like the internet because it has cats. it's better than phones, which traditionally had no cats.
Assuming you already know jQuery (since most everyone who works with js does), what benefit is there to learning YUI over the jQuery based frameworks? It seems like you would have to learn something new and complex anyway.