Rules is an effective tool for Drupal site builders, that many users find dramatically improves their Drupal skills. The Drupal Learning Curve: a configurators view documents this effect.
Rules, like Drupal, reuses its great features at the most basic level and all the way through to the most advanced. So learning the concepts for basic configurations will have compounding benefits leading to very advanced usages.
The NodeOne Introduction to Rules video series provides a great way to easily see Rules in action and have a guide when learning how to use it. And there is also Learn Rules with NodeOne - Drupal 6 video tutorials.Read more