We are using extjs in our next UI version...the widgets are awesome. The docs are sometimes a bit terse. The examples are usually contrived in ways that completely avoid real world requirements...which often turn out to be quite complex to get working. Stuff like setting the selected item in a combo box that is generated via a JSON object takes several steps, and pre-loading a TreeMenu is laughably complex. It often feels like the goal was to make as much as possible load on-demand, with no possibility to use those widgets without the lazy-loading features...which makes it much slower to use in every use case I can imagine...we wrote our own tree menu that loads all items in one go instead of fighting with it anymore. Also, sometimes things change pretty dramatically between versions.
But it's the best thing we've found for our needs. It looks great, runs fast, and the developers offer reasonable commercial support.