You could also download a copy of DNN from the downloads page and try it out on your local machine - this way you can play around with the site and see what it has to offer that might be different from other CMS products you've tried out...
Is there something specific you're looking for with regards to a CMS product? In general DotNetNuke (aka DNN) is a very robust framework that allows you to do whatever you want - but it also makes an assumption that you are either willing to
a)purchase modules from the various publishers out there or
b) you are comfortable enough making customizations/ or new modules to suit your specific needs.
For some people, the core modules that are released as part of the DNN framework suffice for what they are trying to do while for others, they need to have much more expanded functionality which the people create by themselves..
Sanjay
Like the poster before me mentioned, check out the show case too - this would give you an idea of the diverse range of sites that are using DNN.