Tracking is weird. One hand it's gross and I don't like it. I think because who knows who is looking at what you're looking at. On the other hand, as a consumer I actually find the ads more valuable, and I sometimes buy items advertised to me. And as a content creator, they generate much more revenue.
I think there's a line that's often crossed. If there's no (or light) personalization, the ad shown will be relevant to the content it's next to, and maybe not relevant to me. If there's too much personalization, the ad shown will be relevant to me (or my browsing history), but not really relevant to the content it's next to. I would be less creeped out by personalization if the ads fit on the page; so when I view a news article about the Pope's visit, probably don't show me retargeting ads for a service provider whose API docs I was looking at yesterday; but if it's a technology site, then go ahead. Also take a lesson from Target; just because you can figure out I'm pregnant, don't make it obvious you know, put some deliberately mistargeted things in there too, so it looks more random.
Google ads used to be valuable because they were based on the content of the site you were currently watching. Doubleclick (later bought by Google) and others already did the tracking thing and didn't get mileage out of it. Retargeting (i.e. following you around with ads for something that you may buy with a more than 3% chance because you actually searched for it) is the opposite of that, purely tracking-based and mostly annoying.
