The YUI code is well organized, well commented and easy to read. As with most of these projects, one tends to feel that the engineers working on them have too much time on their hands.
In a big project, simple, readable code that is flat and easy to adapt is much more powerful than gee whizbang widgets. I can figure out the candy for myself as needed. Debugging JS is hard enough without having to wade through reams of clever code.
