For a good laugh, hunt down the "Your interests" / ad preferences page while logged in to Google. Out of the 18 things Google believes I'm interested in, maybe 4 sort-of fit, the other ones were mostly stuff I'm pretty sure I never even searched for once.

I'm not sure if Google's staggering incompetence in all things user-facing is a pro or a con, though. On the one hand, you could make the Stasi argument and say that people just think the company actually has meaningful information on them - but on the other hand it clearly has the potential of being even more dangerous, especially considering the fact that Google data is presumably being funneled straight into governmental intelligence as well.