@rfletcherEW talk at GR8Conf US 2015 is a chant to write Spock tests with clarity and expressiveness in mind. Please watch the talk or at least read the Idiomatic Spock slides. It is pure gold.
I specially liked the concept of writing test with two persons in mind, the person who reads the tests for the first time and the person who has to fix the test after a failure. With the latter in mind I learned how to use Unroll better and how to write tests which output a more clear diagnostic information.
Moreover, it is the first time where someone gives me a clear explanation about when to use when/then or given/expect.