You also have to ensure that the machine is learning what is actually necessary.  I recently bought a new Chevy Silverado and the amount of SPAM I now get about Chevy Silverado's being on sale is insane.  I just bought one... why the hell would anyone think I'm in the market for another?  Stupid.  Just plain stupid.  The machine learned how to be a total moroff ("Moroff" is worse than "Moron" because it's more off than on :D).