[nkurz]

I'd add an alternative:

  #3: Suggest items that they are likely to really love.

This is subtly different than predicting what the user is most likely to like. To optimize with RMSE scoring, you are better off suggesting a sure "4" than a risky "5". For buying an expensive item like a car or a stereo, the safe bet might be a good approach. But for books, music, or movies --- easily sampled, one of a series --- I'd be much more excited by a system that can predict A+ items with even 25% probability than one that offers up straight B items with 80% consistency.