Earlier this year, the University of Baltimore (UB) conducted usability testing on Drupal's admin interface with web developers already familiar with other CMS in the same space, but with no prior Drupal experience.

Users mostly struggled with nomenclature, key functionality burried within dropdown lists, and understanding that the administrative menu is overlaid on top of the actual site, leading to confusion about how to preview work and test links that were created. One developer commented "it's hard to visualize what the structure of the site is at this point, what it looks like". UB recommended that the default Drupal theme should visually distinguish between admin and non-admin controls.

The video resolution is low, but worth following if you're already familiar with Drupal's admin interface - http://video.google.com/videoplay?docid=-3942663057311698508&hl=en

Additional findings, coupled with an earlier study done by the the University of Minnesota can be found on Drupal's site. Some interesting comments followed as well.