I used to use YUI heavily, but not so much any more - I really rely on jQuery to get most of my work done. I still use YUI for the heavy lifting stuff, like creating dialogs, calendar widgets, that kind of thing.

But I found I was really over-relying on YUI stuff just because it was there. For example, we use YUI buttons throughout our app because I figured them out and thought they were cool and looked nice. Which is all true, but it adds extra overhead to pretty much any form we create now.

I try to keep to jQuery as much as possible and only delve into YUI when strictly necessary (though I really wish jQuery would bundle an onAvailable method into the core.)