I worked at company that did text retrieval and we met with a startup that was founded by defectors from both Bing and Google and who thought both of them sucked 10 years ago and specifically were unable to develop a true personalization service for business reasons.

One problem they pointed out was a hard limit of about 70% precision for the first result as you never really know if a person is querying “jaguar” if they are interested in cats, cars, game consoles or something else.