With the latest performance enhancements and great dynamic, functional and meta-programming features Groovy becomes a very good choice not only for scripting, but also for big and complex application building. Long term complex application development requires extensive unit testing utilization to allow moving forward without breaking existing features. Groovy plays nicely with powerful Spring framework, which can be used to make application easily extensible and unit testing friendly.
I've faced a problem with unit testing web flow controllers
There's an existings WebFlowUnitTestMixin for unit testing webflow in GDK, but no word on how to use it in the latest Grails 2.2.0 official documentation.