This project I've been involved with over the last 6 months is an example of how EXT JS can be used to develop applications in the financial services industry.
This is an application for trading environmental Comodities such as Carbon Credits. You need to be a client of Markit to have access to the application but I've posted a short intro and description about it on my website.
Markit Environmental Registry
Rich Application for Carbon Market Transactions
I have to say I was very impressed with the guys who put together EXT JS. It was highly intuitive, clean and easy to follow (you'd be amazed how many other oo frameworks are unusable because of over-engineering and poor choices when naming classes / methods / properties, thankfully when using EXT you can expect the classes and method names to be intuitive and to do what they say).
There were several major releases during development (we went with release 3.2.0 in the end). And the EXT development team actually implemented a bug that we reported with grouping grids in a short space of time.
I'd say the only downside to the EXT JS framework is how hard it is to customize the look-and-feel, but there is no easy workaround here. This is a framework that is targeted to building APPLICATIONS and I think the approach of making the framework as close to an enhanced windows UI as possible so that the EXT UI can easily be confused for desktop applications has been correct. You can always put a banner on top and apply your own branding there.
One big cheer for EXT team. Hip hip HURRAY!!!
Steven de Salas
Web and UI Developer