I run an advertising network for Microsoft community sites (Lake Quincy Media), which typically servers 60 Million impressions per month these days.  I'm been using CountryHawk to provide geotargeting as a service to advertisers who want to avoid showing their ads to users from certain countries, or who wish to limit their ads to a particular subset of countries.  Since implementing CountryHawk (and BrowserHawk - see my BrowserHawk review), I have not seen any noticeable performance degradation on my system, and the advertisers who have taken advantage of geotargeting have been very satisfied.  What else can I say?  It just works.  In addition to the product's features and usefulness, I can also attest that cyScape's support is very responsive and helpful, as I did have some questions for them regarding a unique application configuration I was using.