Because of this, I don't see many ads. But I have been an amazon customer since 1999 (according to what they say on their website when I'm logged in.) Looking at what they recommend for me, this personalization stuff is crap.
In music, Amazon recommends bands I never listen to like Montrose, Metallica, and the Doors (and to be fair, some people I've never heard of so I guess it is possible that I would be interested in them. Greta Van Fleet? William Patrick Corgan?)
In books, I do like scifi but they recommend a bunch of books with spaceships shooting each other on the cover - not what I have ever been interested in.
In the "humor and entertainment" section of books they do list some books that I would be interested in but, strangely, none of them are "humor" but are all academic books about videogames (which I am interested in). Even here the recommendation engine is very unsophisticated because in between academic books on videogames there are books on the art of Zelda and other coffee table books that I am not interested in.
And the first book in their recommended children's book section is 1984. (and I don't have any kids any way).
If this is the best they can do with 18 years of tracking my purchases then I am not worried.