Maybe "creepy" - I search for an item, and Google shows me ads for that item for the next month; I keep searching for that item, and Google learns to tailor its results to what I want.
Some people find this useful. Other people find it creepy.
I find the creepiest search ads are ones that direct me right back to the context of a site I was already surfing on a few minutes earlier. E.g. I was looking at the Amazon Sci Fi book section for example. Then, via google adsense on some other blog page I'll now see an adsense ad for Sci Fi books on Amazon. This has happened to me a couple of times now.
Some people find this useful. Other people find it creepy.