Go.waka.ca is a canadian real estate search engine that remodeled its user interface with Ext's goodness. It was implemented using a lot of Ext's widget including the BorderLayout, the Grid, the Tree & a custom paged View.
We have developed an application using Yui-ext (even if we have not upgraded to the 1.x version, yet) and MochiKit. [2]
The aim of the application is quite "original": allow everybody to securely store private information online. You can find all the details about our application on our main site [1],
We didn't use many "standard" yui-ext widgets, but most of the application interface is done using the yui-ext DomHelper class.
Another interesting feature is that the whole interface is dynamically generated by the JavaScript code; once loaded the index file, the server provide only (encrypted) data to the client through xhr requests (handled with DWR [5]).
As an added bonus, all the application code (and build script) is also available for inspection [3] (with a reference licence [4]).