Here is my take on PHP Nuke…..
And, this is not from direct experience with that particular CMS.
I have used DotNetNuke (The .NET version, basically) and other CMS’. There is quite the learning curve if you have not used PHP or CSS before.
Right now I am in the midst of creating a site with Mambo and it has been quite the task. And I do have experience with web development (mostly xhtml, html, and asp / asp.net instead of php). While the CMS’ are very fully functioned, you still have to find workarounds to get your site to look and feel how you want it to. Yes – I know that there are templates to pull from, but you will still want/need to customize it. And that takes time… Usually more than you thought it would.
For example (problems you can run into)…. I had a real problem tying in remote flash objects to my site. Took me about 4 hours to figure it out (even with forum help) and I think I lost some hair 
(Another Example) I changed a template and then the site wouldn’t quite span properly… So I ended up creating my own template from scratch (a longer process).
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OK. One of the real issues with CMS system is: Being Search Engine Friendly. There are typically some 3rd party tools to help out, though. I am not sure what PHP Nuke has, but you might want to research before putting your whole website into such a ‘wrapper.’
Just my 1 1/2 cents.
Hope this all makes sense because it felt more like a rant than help
kw