I can't recommend the discussion between Kent Beck, David Hansson and Martin Fowler enough. They look at how and where it should be used and in particular discuss their approach to mocking which I feel is an often misunderstood tool.

Part 1 - https://www.youtube.com/watch?v=z9quxZsLcfo

Part 2 - https://www.youtube.com/watch?v=JoTB2mcjU7w

Part 3 - https://www.youtube.com/watch?v=YNw4baDz6WA

Part 4/5 - https://www.youtube.com/watch?v=gWD6REVeKW4