I'm gradually converting a back-log of existing tests to Spock ... and some of them convert so beautifully, it hurts. Here's an example:
Before (Java and TestNG)
What a difference; the data-driven power of the where: block makes this stuff a bit of a snap, and you can see in once place, at a glance, what's going on. IDEA even lines up all the pipe characters automatically (wow!). It's obvious how the tests execute, and easy to see how to add new tests for new cases. By comparison, the TestNG version looks like a blob of code ... it takes a bit more scrutiny to see exactly what it is testing and how.
In addition, the propertyMissing() trick means that any property (including public static fields) of Calendar is accessible without qualification, making things look even nicer. This is what they mean by writing an "executable specification", rather than just writing code.
I can't say this enough: using any other framework for testing Java or Groovy code would simply not be logical.