What we should all want (yes, even you, dear reader!) is a system that not only helps us find what we already know we like, but what we might not know we like because we haven't tried it yet -- a sampler, if you will. (Or as my friends know it, the "mixed grill". Tasty!) At the very least it should find representative entities for each group, and have a user try them out one at a time; that way they don't have to explore everything to get a better feel for what they'll like.
But there's also the idea of randomly introducing items into the preferred list without telling the user -- they'll just think it's a mistake, but maybe they'll give it a try, and maybe we can help friends discover how different they are from each other in addition to what they already knew -- how similar they are. Also, consider that tastes change over time; a long-lived system shouldn't continue to suggest that you listen to the same music you liked when you were a teen; it should constantly force you to give other music another try, to see if your tastes have changed. You can't do that without a little randomness, a little slyness.
The "related items (implicit)" system we use on this site is a case of only finding what you already know you'll like, and as such, doesn't help people browse and sample the whole thing. Then again ... for a stand-alone site, that's probably not what most of our (rare) visitors want anyway.