I just returned from my Web Mining class. A student made the point there that never in his 10 years of using Amazon.com has their recommendation system ever suggested something that even made sense to him, let alone led to a purchase. He has also rated some 4,000 movies on NetFlix in hopes that he could find some new movie that he would enjoy. He has a very low opinion now of recommendation systems.
I would agree with him, though my opinion is not so strong. Never has a recommendation system worked for me. Either the recommendation is obvious, I bought a CD by artist A so I am recommended CD's by artist A that I am already aware of or the recommendation is for something unrelated and unuseful. The only time that the recommendation system seems to work for me is when I am new to a specific area and am looking for the most popular books in that area.
Recommendation systems have a difficult task. Computer systems cannot understand the human mind. Everyone makes a purchase or likes a specific book for a reason. Those reasons are different between people making the same purchase. How can you then recommend the same thing to two different people that bought that book for different reasons? It does not make sense.
The one case that I thought of where recommendations have worked for me is Pandora.com. That music recommendation system has led me to learn of new artists and songs that I like. It made many wrong song suggestions, but I like enough music that eventually something new was played that I liked. Not through a wonderful system, but more of trial and error in a genre.