Yes, we had some conflict with pf. The GUI wouldn’t display properly and who knows what internally?
The reason I was told by avast staff : PF provides antimalware protection so there might be some conflict, if not implicit, and you may not use 2 resident antimalware stuff at once. Check the pf features: http://www.privacyware.com/personal_firewall.html
There might be people who are absolutely fine with PF. But you asked for conflict and I have experienced it.