Drupal content management system is the third most popular content management system driving the Internet. It is considered to be technically the most brilliant but that is mostly due to behind the scenes technology. It is also considered to have enterprise level security. Under the Obama administration it was selected to power the White House website.
From an end user perspective the brilliance is not always as evident. The user interface is less intuitive. There is more need to edit files manually. Although improving with respect to extending with new or updating installed modules and themes, there is not a very well developed ability to browse available modules from the administrative pages. Instead you will still have to browse the drupal site and the copy and paste a theme file location into the administrative install page, or even download the tar.gz or zip file, and unpack and manually copy and paste to its location in the drupal directory tree.
With respect to multisite installation capabilities and also capabilities to develop your own views and fields, Drupal is ahead of WordPres and Joomla, but the learning curve to be able to use these capabilities is steep. But for those with sufficient knowledge of PHP, Java and SQL, the flexibility is probably extremely rewarding. The multisite capabilities certainly have been a reason for me to run my sites on Drupal 6 and Drupal 7 for many years. And those sites have generally run without any problems with very little attention.
Development of Drupal seems to have slowed, with many modules and themes lagging quite a bit behind the core development.
To obtain Drupal or to extend it with modules or themes: