Drupal is the world's best content management system.  Very large sites like The White House, The Economist and The Los Angeles Examiner use it and so do we.  While suffering from a steeper learning curve than Wordpress, or Joomla, Drupal's infinite scalability and module programming possibilities make it the killer platform that can allow you to do literally anything you can wrap your head around.

Drupal supports:

• Community web portals
• Discussion sites
• Corporate web sites
• Intranet applications
• Personal web sites or blogs
• Aficionado sites
• E-commerce applications
• Resource directories
• Social Networking sites
• Electronic commerce
• Blogs
• Collaborative authoring environments
• Forums
• Peer-to-peer networking
• Newsletters
• Podcasting
• Picture galleries
• File uploads and downloads

And it supports anything you want to write a module to accomplish as well.  If you can think it up and it hasn't been done yet, Drupal will allow you to do it.  Drupal is also massively scalable allowing it to handle hundreds of thousands (and with the right implementation) even millions of users.

The White House uses Drupal because it supports flexible application development and deployment, scalability as well as the standard CMS benefits offered by other content management systems.  We use it for the same reason.  We've got plans for Drupal and Drupal has plans for us.