I find it more of an invasion of privacy than cool. Can't buy a wrench without someone knowing about it and then throwing a dozen adds in front of you after because you got a certain product.
Well, in my opinion, it's terribly efficient and most likely tenfold more effective than blind advertising. Think about it, if the internet didn't know your interests, your location and what you buy, you would be getting extremely random ads, most of them not pertaining to you. Say you're a college student and you might be getting ads for i don't know, male enhancement, house improvement products etc. Stuff that you would never think about buying at that time in your life.
On the business side, all those advertising dollars are thoroughly wasted on that individual and the advertisers get no return on that wasted money. That's just bad business. Also, that individual will probably be not only extremely annoyed by seeing ads that he can't relate to, but confused as well. Wouldn't you rather be seeing something that at least in some respect relates to you?
On the flip side, adaptable advertising is extremely efficient because you are reaching a market that, based on their actions (an extremely reliable source for finding out what interests them), will have the highest chance of responding to an online advertisement because you're "catering" to their interests. It's sort of like a system of recommendations, not unlike the recommendations youtube gives you based on past videos you've watched. It's kind of like saying, "oh, so you've bought that? Then you might like this!!"
The fact that the internet somehow is able to dissect all this information on everyone in an instant and "recommend" relatable products in the form of advertisements, i'd say is pretty cool.