I have been using NucleusCMS for the last 5 months after having spent many years downloading/evaluating Blog/CMS apps.
Why I settled on Nucleus can be found here: http://hcgtv.com/item/224
I'm in the process of investigating the integration of a forum into Nucleus for seamless operation. From what I see of punBB so far, it fits very well into Nucleus' principles of small code base and light on system resources.
Rather than re-invent the wheel, maybe we can work together to make Nucleus/punBB work well together.