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:
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.
— Dries Buytaert