You'll most likely see the most personal firewall usage as part of your endpoint security. Many vpn packages include them now for the end clients to further bolster their security and make them less likely to be used as a go between for traffic into your network.

They can also be a good idea to stick on machines that take advantage of wireless connections to filter unwanted traffic. This can also be accomplished by taking advantage of ipsec and it's packet filtering capabilities.
No.
Personal firewalls are garbage... in a networked environment no system can ever be trusted to manage its own security.

Systems that are mobile must be on their own network segment from the internal trusted LAN.

IPSEC is different in that it is centrally managed and not "personal". It still shouldn't be on local systems though.

cheers,

catch