You know when a piece of software is mature when it starts being adopted by financial services organizations. ING Financial Services recently moved a number of sites from Oracle Stellent to Drupal. Among these sites are http://ing.us, the main portal for their US market. The driver behind this migration was to move to a platform that was more dynamic and provided faster time to market.
During the initial launch of the project, ING had no one that knew Drupal, MySQL or PHP. They brought in external help to do the site development. Today, they have an in-house team to develop upcoming Drupal sites. Many large organizations follow exactly this trajectory.