Well, that's definitely one for the "suspicious until replicated" bin. They generalized all that from asking people to buy a single song off iTunes. Off the top of my head, people are better judges of music quality in their preferred genres. There's also not much to distinguish "a song bought from iTunes that I like" vs "a song bought from iTunes I think you'll like" in terms of thought invested, allowing smaller effects to dominate.

I suspect if you broadened the scope even a little, the effect would reverse. How about beverages? Alice likes coffee and hates whiskey. Bob hates coffee but loves a single malt. What should Alice and Bob get each other for Christmas?