I use Digg but I found the recommendation engine pretty poor. The problem I had with it is it's very simplistic. If you and user X have both digged up a story, and then user X diggs up another story, it concludes that you probably like that story too because you both agreed on the first story. This tends to be not true all the time. Ideally they should try for more of a netflix-like system that evaluates why certain people like certain types of stories, but that would be much more effort to implement