I have some experience with using DotNetNuke as a CMS. It’s free, open-source, and has pretty good engineering. There are plenty of add-on modules available from sites like Snowcovered. Some of the UI elements are irritating: like a lot of early ASP.NET sites, there are too many postbacks, but it works well in general, and is easy to manage.
Community Server is targetted at a different market, and is ready-made for a typical portal site, with blogs, file downloads, forums etc.; whereas DNN is more like a toolbox, Community Server is one of those 5-in-1 screwdriver/drill combos. The asp.net site now runs Community Server, not surprising, since both are Microsoft-driven.
CMSs like PHP-Nuke are hopelessly out of date and riddled with bugs. In 2006 there is no excuse for having any SQL Injection vulns in web applications. Just do not execute raw SQL from the code: use Stored Procedures and parameters!
Joomla! is a fresh effort built on the base of Mambo, and is so far free of major bugs. It supports a flexible templating system, and has a bunch of nice features out of the can. It is, however, written in PHP, which is not very secure; PHP is seen by many as the scourge of the internet.