For us, about midway through our upgrade, we realized that we badly needed a tool to help us verify that what used to work in the Ext JS 3 version still worked in the Ext JS 4 version. Doing the testing manually for each feature and API method would simply have taken way too much of our time. After evaluating a few popular options, we realized that none really fit our needs, so we started to design and write our own tool: Siesta
Looks nice for unit testing where you write the tests programmatically. Not sure how it would work with a recorder and automatically generated IDs. Price tag of $300/developer is steep not to do everything.