I'm pretty sure I read about a TechTV stunt where they pitted OSX and XP's firewalls. They gave out the IPs on air and waited. Within 5 minutes, I read, the XP computer was bluescreening while the OSX mac was fine. I believe they opened some utility and showed that people were trying to get in, but were denied. I don't have it on, but I do here it's pretty good in terms of a software firewire. Though if your data is critical enough, you should have a hardware based firewire (i.e if you're running a business or working on the CIA's hovercar).