Just evaluating the complete toolkit including designer for a new project. In the past I've used Selenium for testing an application created with another library (similar to Ext JS). I had the problem of defining selectors for Selenium to interact with the app, and the same seems to apply with Ext JS as dynamic IDs are assigned to items.
I would really like to use the Designer. It's the main reason for choosing Ext JS for this project instead of the other library. However, if it's going to make testing difficult, or use of the designer is only for the initial creation of the UI and not for maintenance, I might have to think again.