That depends on your use case. Huge numbers of people (e.g. people at work) use VPNs, and their 'geolocation' could be wildly different than their actual location. If you're an IBM employee (what...quarter of a million people) on the VPN, you look like you're in New York someplace. At my current employer (80k), most of us look like we're in Minneapolis, even though I'm half a continent away. If you're, say, targeting ads based on city/state level GeoIP, that's a lot of misdirected ads.