Evaluation of Javascript Libraries. My Opinion It has everything you might need, but with different levels of refinement. The widgets are quite powerful, yet they don't seem to have the behaviour that you would expect from them, e.g., the calendar widget. One area where I feel YUI is weak is DOM manipulation.
All the widgets that I've seen require extra s to render themselves. This is good (because you define exactly where do you want the widget to render) and bad (because it requires modifying the underlying HTML document in a non-trivial fashion). Docs are excellent. Finally, widgets don't work in Opera. (ADDITION: Hello, this is Nate Koechley from the YUI Team at Yahoo!. Dojo - DojoToolkit.org. Script.aculo.us - web 2.0 javascript.
Script.aculo.us: How to easily AJAX your site - Download Squad. Dr. Dobb's | The ZK Framework | January 15, 2008. Open Source AJAX Frameworks with Server-Side Java Support - Manageability. OpenLaszlo.