Recently, two projects I have been working on are using Drupal. Drupal is a content management system which includes plenty of flexibility and extensibility. That means if you want a web site with all the bells and whistles and have it handle your special requirements, Drupal is a great choice. It is easier for web developers to customize than other content management systems. You can have a great web site at a lower cost than building a completely custom web site or application.
Although the Drupal community calls it a content management system, you can also view Drupal as a rich web framework. Learn more about working with Drupal.
With these advantages, it is hard to see why you would not use Drupal. Thoughts to consider when using any software include: