We use Drupal for the main corporate brochure and community areas of our website at http://www.tableausoftware.com
It has allowed us to add on a multitude of plug-able features and customize UX to meet our needs far beyond what we could do with either a more bare-bones platform like rails or django better suited for apps.
We still have room to grow for performance tuning, but we have millions of hits a year without a problem. We've made use of Solr indexing to improve search and allow faceted integrated search for site content and knowledge base and support forums. Additionally, our team has been empowered to create content and curate the site without development hassles.
Lastly, with recent community focus on tools like Features and Context we've been able to manage more risk free deployment and workflow/environment management in addition to improved site architecture.