I am no expert but PF didn't gave me any headache at all (well, almost, it blocked conn at first try but I managed to fix that pretty fast). But I use internet connection only a couple of minutes per day ...
Ask Paul in freewares part 2. He just posted a link to new version of it ...
Well I waited around until they released Avast 5.1... So uninstalled Avast 4.8, Outpost 2009 Free firewall and installed Avast 5.1, followed by Outpost Security Suite Free. Very nice combo. By installing Avast first, when I installed Outpost, it recognized Avast and did not install its own anti-virus module, but it did install what appears to be a basic, but decent anti-spyware realtime module.
The combo of Avast and Outpost seems to be working well and I haven't (yet) seen any performance hit.