I have to agree about the flexibility of EE (multiblog, template customization, custom fields). And compared to pMachine (even the pro version), yes, i think EE is MUCH better.
Expression Engine has its Core version too, right? So, if there is only a single account in pMachine, why not switching to EE core? 
Anyway, I am also interested in integrating punBB to Expression Engine. Is it possible? Anyone?