Being very fond of mock objects, I have pushed the concept to the limit a few times. In the beginning I thought that ALL testing of a units interaction with its collaborators could be tested using mock objects. But I have run into a few situations where it can't (I think). I have written a small text/article about it here:

http://www.jenkov.com/training/unit_test/mock_testing_isnt_always_enough.tmpl

What is your experience with mock object testing? Good? Bad? What limitations have you run into?