I have one of these http://williamknowles.co.uk/?p=16 with a 2Tb external usb drive hooked up to my adsl modem and pfsense wan interface.
This will save more packet capture data over a longer period of time than the the pfsense packet capture which hangs after a period of inactivity relkated in part to the default auto log off setting seen in System, User Manager, Settings tab, Session Timeout which is a default 4 hours.
Might be useful as an alternative packet capture but notethe Rpi only does 10/100 nic speeds so no good on the lan side for most networks but useful for slower broadband connections.
fwiw.