Forrester Research published two reports on Drupal. A 6-page report on Drupal itself, and a 15-page report that compares blogging platforms (including Drupal). I don't have access to the reports but here is the executive summary that was published on their website:

Drupal provides an open source solution for integrating blogging into an overall community or content site. That integration allows authors to take traditional blog publishing tools and extend them into discussion boards, wikis, and traditional Web pages. Drupal's biggest disadvantage is that its interfaces are difficult to configure and use, although there is a large user community supporting the platform. Companies should use Drupal if they have open source experience and want blogs to be an integrated part of a publishing and community platform.

Last February, Charlene Li, a principal analyst at Forrester, invited me to participate in the screener and information gathering process but I was too busy at that time. It was around the time that the Open Source CMS Summit in Vancouver took place. Fortunately, Bryght was able to answer their questionnaires.

More information is available at Charlene Li's weblog where she wrote about Forrester’s blogging platforms evaluation. The figure below is taken from Charlene's blog:

Forrester blogging platform
Source: Charlene Li's weblog. © Forrester Research.