Every day for the past 5 days Amazon has sent me notices about Photoshop CS5. OK, I admit that I checked out the upgrade price once (turns out I'm not eligible for the upgrade as I only have CS). But really, I don't need to be notified every day. </end unrelated amazon rant>

Amazon thinks I would like Eminem. That's just so wrong on so many levels.

This mystifies me constantly! Like I read a historical novel recently and rated in three stars. Suddenly all of these books about America's history started getting recommended to me (like a biography of Betsy Ross [?????]), based on that lukewarm rating--and the book wasn't even set in America!

I wishlisted a cross-cultural study of infanticide. Amazon decided I needed parenting books.

Their formula might need some tweaking, ya think?

Amazon is recommending recent editions of books I bought 7 or 8 years ago.

Their system is almost entirely based on a simple "people who bought X also bought Y," with few refinements. More complex algorithms which take into account all of your ratings (and comparing with other customers) are too computationally time-consuming to be economically feasible, given the huge number of products and users.

Even last.fm, which does much more interesting analysis to generate recommendations and neighbors (similar users), simplified their system a few years ago because it was becoming unmanageable as the userbase grew. But with that change there was a noticeable decline in the quality of matches.

Amazon recommends several books I worked on, but only since I bought an extra copy of one of the other books I did.