I haven't personally used it in on a live site but I know plenty who have and they all rave about it. I think they are all on version 4 though, but I could be wrong.
If you look around the net you are bound to come across hundreds of sites based on php-nuke, and the can't all be wrong.
I have ve had a play with it a little, and from what I've seen it's very good. Of course I just went and built one myself as I needed a project. 🙂
-- Nick Gushlow.