There’s a huge 92-page (!!!) mutl-blog software review done by Robin Good. Among other systems, it also covers Drupal and WordPress MU (Multi User).
"Group blogs leverage the potential of immediacy and ease of use of
traditional blogs while matching them with the facilities needed to support solid contribution of multiple writers. "
On another note, Tangent Mobile has also put up a review of Drupal and WordPress – 2 of my favourite CMS/blog software. I find myself agreeing with the following comment in particular
"The drawbacks of open source are that you need to be careful which modules you use. Some can clobber the other and can break the system so testing is important"
Once it’s been set up and is nicely running though, the memory of the initial hassles fade away very quickly