Drupal supports open, API-based integration with any system that supports the same for it's data access. Because Drupal has been deployed multiple hundreds of thousands of times, these integration points have been tested to a far greater extent than any proprietary system on the market.

Drupal has a very solid, public roadmap around major revisions, and best practices for upgrading between major releases. In addition, the very nature of open source software and its communities fosters innovation. They are often far ahead of proprietary competitors in providing cutting-edge technological advances into the very latest technologies. For example, Drupal was the first CMS to offer RSS feeds by default and Drupal 7 will be the first to offer RDFa as a core technology.

There are thousands of firms located all over the world that have exceptional experience in creating and modifying Drupal sites. IT professionals with LAMP experience as well as programmers of other languages and systems (Java et al) can easily get the hang of Drupal and PHP and become valuable assets to your project.

Job search engines like indeed.com show very strong growth in Drupal job postings, which in turn is driving many individuals to learn Drupal, or to migrate from Drupal consulting to full time work within organization.

In terms of implementation, there is no difference between open source or proprietary software products. Access to experienced technical resources, product architecture, and the complexity of technical requirements are the key drivers of the success of any software implementation project. Effective project managers is critical to identify project milestones and lead technical resources to hit those milestones according to predictable, agreed upon timelines with business stakeholders.

In addition, a mature open source product, like Drupal, has the benefit of hundreds of thousands of

The LDAP Integration module (http://drupal.org/project/ldap_integration) provides three very nice features:
1) Authentication, which allows you to log in using your LDAP information
2) Groups, gives you roles according to your LDAP groups
3) Data, which allows you to read and write access to LDAP data

Drupal has been used for years as a platform for pulling together communities.