That said, even as a non-statistician, it wasn't hard to imagine the amount of information Google could infer by focusing on an individual. I had a data set exceeding 50k searches with the resulting click-throughs on my account. (Plus, they can of course leverage their massive db to help eliminate anomalies like Whitney Houston)
In addition to the basics like big item purchase history, hobbies, and problems (e.g. sickness); I wouldn't be surprised if Google could predict my relationship status and sexual preferences.
Custom search results are great, but I'm extremely happy to have been given the option to delete that profile. If anything, it is the single largest factor pushing back to Firefox and possibly DuckDuckGo.