My company has been working with ExtJS now since 2007. It took me us a very long time to develop and release our web application (we had an existing version running with an established user base at the time..) . Its finally up and running if anyone wants to check it out you can go to http://v3.logiforms.com/.
You have to sign up for a free trial account. Which sends a 10 day guide on building forms (the unsubscribe link is at the bottom - sorry - blaim marketing for that one).
I had some help from Animal when I first started this project - thanks Animal - without your initial push iin the right direction, the app would not be what it is today.
Its not perfect It works best in FF or Chrome. Its built on 3.4.0. We tried to upgrade to Ext 4.x but were never successfull...