Idealware, an independent non-profit, has published a comparison of Drupal to three of the other most popular Open Source Content Management Systems. Despite a few inaccuracies regarding security and workflows, the report is very helpful, and accurately describes many of Drupal's strengths, including it's superiority in the realm of flexibility and extensibility.

They sum it up this way:

"Flexible and powerful, Drupal is a great choice for more complex site structures - you can define very detailed rules as to what content should be displayed where, and build your own custom content types. It has particularly strong support for Web 2.0 and community functionality, including group blogs and user-submitted content. It's also easy for your content administrators to find and update content.

But Drupal's power comes with complexities.... Most people will want to hire a consultant familiar with Drupal to help them set-up a site, rather than trying to go it alone."

You can read the report for yourself here.