I’ve used JMock for quite some time and I’ve found it to be a great and very useful mocking framework for unit testing. I was never quite happy with the syntax though; especially the part where you specify expectations.  Here’s a JMock snippet illustrating an expectation and a return value from a mocked object

context.checking(new Expectations() {{
    oneOf (department).employees(); will(returnIterator(employees));
}});

In Mockito the equivalent code would look like this

when(department.employees()).thenReturn(employees);

I find the Mockito syntax to be easier to write and understand. I’ve seen fellow developers implement JMock expectations in a way where they actually didn’t perform a useful test, and I think the sometimes confusing syntax of JMock was the reason. I’m not dismissing JMock as an inferior mocking framework, but I do think that the learning curve is a bit steeper. If you haven’t used Mockito yet I encourage you to take a look at it.