My biggest annoyance with retargeting is also buying a car. I don't remember seeing any car ads leading up to my purchase, but ever since buying the car (over a year ago now) my browser is littered with Ford Focus ads. It's not going to do any good to show me ads for a Focus, i already bought one. It's just a waste of your ad buy to show me more ads. Retargeting seems to be, at this point, one of those technologies that has a high potential value but needs a lot of work to realize that value.
Or it could just be that the people who sell fords aren't very smart. Three days (!!!) after i bought my car, they sent me an email promoting a trade-in event they were having, telling me what a good deal i could get on a used toyota at my local dealership if i traded in my car.