I've used Drupal to build a combined public-facing site for an organization that also included internal features for employees such as meeting organization and internal document management. Mostly it worked out well and they are happy with it, and very happy not to have to manage two separate sites and / or something like sharepoint.
The one place I found Drupal to be a little lacking is in its LDAP support. You can use it just fine for authentication, but there are some bugs in the modules that try and bring LDAP groups down as roles, if this worked more seamlessly I would be completely on board with the "Drupal is enterprise-ready" sentiment, but it's not there yet.
That said, you can do a lot with content types and taxonomy-based access control to customize what content is available to what roles, something that is a requirement for most intranets.
