I was not impressed with Mambo either. My biggest gripes are
1. Not XHTML compliant, in fact, it is a mess.
2. Upgrading after styling is a pain because if you upgrade it, all your work is lost. I know there's a way to work around this, but for most people wanting something done right away, you are on your own.
I also have used and installed Typo3 for an intranet, and that has a steep learning curve. I never cared for the scripting language it uses, but once I got used to it, I could hack it.
1. Mambo is XHTML Transitional "Out of the box". There is tag soup in there (tables and gubbins) but this will be fixed in the next version 4.5.3 - which will be released probably next September.
2. Mambo uses a template system - If you use a valid CSS XHTML layout you don't have to upgrade anything - this will be replaced by Pat Templating in the next version.
Mambo is really easy to understand unlike the steep learning curve of Typo3 and isn't just a simple blogging system. The community is huge and very helpful with many users willing and able to make a clean product like PunBB.
PunBB looks great and I hope there will be an integration with the best CMS and best Forum in the near future.