In a great essay titled "You really, really need to do Usability Tests," Lukas Mathis makes the case for testing your application's user interface with real users, instead of guessing what UI will work with your target audience, or relying solely on using designs that have proven succesful with other projects.

If you're a developer writing software, do you perform such tests with real users? How do you plan, execute, and evaluate such tests? Do you use software -- like Silverback (product website; my mini-review) -- for testing? What best practices have you come up with?

And if you do not run user tests, what's keeping you?