Yes! I used to work for a company that solely used DotNetNuke for it's CMS. There are actually quite a few similarities between the two, at least in the concept of how pages are edited. They both have:

-Themes(c5)/Skins(dnn)
-Editable areas within a theme to which you can add blocks ("modules" in dnn)
-In-context editing

However, I never really liked DNN. It may have just been our servers but it was almost always excruciatingly slow, which I hated. Seemed like everytime you changed a dropdown it would start loading some ajax something or other. The UI was not pretty at all. For me, it was a good CMS in concept, but just wasn't fleshed out just right. But hey, it runs on a Microsoft platform...what do you expect? :)

I came across C5 towards the end of my working for that company. To my delight it had everything I liked about DNN and nothing I didn't. Plus it had a MUCH prettier and intuitive UI. Of course, it's PHP/MySQL as well which I prefer a million times over dotnet.

In terms of themes/skins - I like the way C5's work much better. I remember DNN adding a bunch of junk code into the skins - there was a lot you didn't have control over. In C5 you have much more control over exactly what goes into your templates.

Lastly, extending C5 seems much easier to me than extending DNN. I watched like 5 hour-long video tutorials on how to build DNN modules, and still never really built one that did anything useful. With C5, I was able to build my first block in an hour or so without even much direction.

So, that's my take on the two.

I remember usinghttp://www.cmsmatrix.org/ to choose between CMS systems, which lead me to C5.

about cmsmatrix, it looks like it's not up to date anymore..

UTF-8 Support, Content Reuse, Document Management, Captcha and probably a few more things are supported now. They are still at version 5.0.0

How can this be updated?