More in msg 6 in this thread:
[webmasterworld.com...]
The benefit to this is that it will pick up very specific queries at a lower CPC and higher CTR.
The downside is that you will miss maybe half of the people that search as they will not put in the area.
I have a UK based campaign that is costing half in CPC and has double the CTR, not bad but it does take time to research and implement well.