A recent head-to-head comparison of Open Source CMS products credited DotNetNuke with "high usability, a large number of stock modules, and a penchant for commerce." The article compares DotNetNuke and other offerings such as Alfresco, Drupal, Joomla and Plone: 

Head-to-head: Open Source CMS

The review identified authentication, workflow and versioning as shortcomings. Of these authentication has already been addressed in 4.6 and the remaining features are in the pipeline.

We also have some exciting things in store for DotNetNuke 5.0 which will make DotNetNuke even stronger (OpenForce '07 attendees will get a first-look).

Update: This article was also published by InfoWorld with helpful comparison grids:

Open source CMSes prove well worth the price