You can search around--the advice on firewalls is frequent, but the decision to open up has to be yours. So far, I haven't been burned when I leave it off, but then I don't keep much of interest that isn't already shared.

The default firewall setting for LW on OSX is buried in the System Preferences->Sharing->Firewall->New->Gnutella/Limewire (6346). That's the only one you need to checkbox. If you go with a different port number, just create a new one instead.

Sure like those iBooks--coming up to our first anniversary with mine (extended warranty helps keep me happy).