I am a big fan of the Test tool, especially since we got Browse Anywhere. I find it an essential tool for writing custom extensions - believe the in house devs use it for testing the core tools too.
I use it on joins where I expect complete coverage of one or more of the inputs and verify that Left or Right outputs contain no rows. You can attach filters as precursor steps to validate data rows as well.
For column checking the Field Info tool is invaluable. Again often with the test tool.
Main downside of the Test tool is no pass through, can achieve some good verification checks with the Message tool as well.
