I have looked at and reviewed quite a few CMS systems. The ones that stood out to me are Zope/plone (python), Xoops (PHP), Drupal (PHP), and Bricolage (Perl). I have installed and used all but Bricolage (more on that later). All the systems have their advantages and drawbacks. And a lot depends on what you want to do with the CMS.

I found that PostNuke's code was a mess. I would not recommend using it. It has also had quite a few security issues in the past.

Zope/plone:

Strengths:

• Very easy installation
• Cross platform (windows, *nix, mac)
• Built-in Database & webserver
• Many add-ons available (called Products in Zopese)
• Large developer community
• One of the few OSS CMS systems with a real industrial-strength workflow engine.

• Zope w/plone has a rather steep learning curve for admins.
• It has a steep learning curve for programmers creating new add-ons (Products).

Drupal:

Strengths:

• Well designed code makes it easy to modify/create add-ons(and run fast too).
• Has strong syndication features (RSS , Blogger API, etc.)
• Theme (templating) system is easy to use and well designed.
• Admin interface is fairly easy to use.
• Taxonomy metadata system is unique
• Well documented on its web site (drupal.org).

• Smaller development community.
• Installation requires manual setup of the database.

Xoops:

Strengths:

• Many add-ons available.
• Has strong developer community.
• Admin interface is very easy to use.
• Easy web based installation.

• Documentation sometimes spotty.
• Many add-ons are of varying quality.
• Engine makes pure CSS layouts difficult (lots of tables built-into core layout).
• Somewhat weak workflow engine.

Bricolage: I have not run bricolage (I have installed and run all the others) due to its strict requirement for Postgres. So take these comments with a grain of salt they are based on using demos for a short period of time.

Strengths:

• Very well designed workflow engine (created to run Salon.com).
• Its in Perl :-)
• Admin interface is fairly easy to use.
• Templating system is switchable (currently HTML::Template or Mason).
• Proven performer in the industry (Salon.com, Macworld.com, etc.)
• Well documented.

• Very specific requirements (requires Postgres DB).
• Installation can be prickly (requires many Perl modules, no binary pre-compiled packages, other specific dependancies)

Another option for you may be Callisto. It uses AxKit (Perl, XSL) underneath but is designed to be entirely wysiwyg.

UPDATE: Added links.