When I tried learning about JMock and EasyMock, I found the learning curve to be a bit steep (though maybe that's just me).
I like Mockito because of its simple and clean syntax that I was able to grasp pretty quickly. The minimal syntax is designed to support the common cases very well, although the few times I needed to do something more complicated I found what I wanted was supported and easy to grasp.
Here's an (abridged) example from the Mockito homepage:
import static org.mockito.Mockito.*; List mockedList = mock(List.class); mockedList.clear(); verify(mockedList).clear();
It doesn't get much simpler than that.
The only major downside I can think of is that it won't mock static methods.