I've used PHPWebsite, CPGNuke, PHPNuke, and now Postnuke.
I really don't like the way PHPNuke is being incrementally taken into being a commercial product, so I worry about its future. Also, it is a resource hog...
I tried "CPGNuke", a PHPNuke fork which promises to use fewer resources, but found out that if you want to do anything with it that's not included in the default install, you basically have to learn everything there is to know about the program in order to convert Nuke Modules and Blocks to work with it.
Their "instructions" (quotes for sarcasm) for converting blocks probably makes sense to those who already know how to do it backwards and forwards, but it's no "step by step" for those wishing just to get ONE thing done. Furthermore, they're on the verge of coming out with a new version (Dragonfly) wholly incompatable with older versions, including modules and blocks, so EVERYTHING is now "wait and see"...
PHPWebsite
http://phpwebsite.appstate.edu/ I found to be very good because, unlike all of the others, it has a very good working calendar module included with the default setup. It doesn't seem to have much 3rd party module development going on, however.
I'm now playing with PostNuke, but again, they've introduced a new Theme Module which is incompatable with older versions. Furthermore, I haven't been able to find a theme that doesn't somehow require the right (3rd) column, which I don't want in some cases.
I'm looking for something more sophisticated than a simple blog. I'm looking for multiple admins, calendar (essential), theming and integrated message board.
Anyone with more experience as to which way to go?