“I find playing against computers depressing. I don’t like losing. I also think it's not so useful practise. Even though computers have become more human in their style ... it doesn't help you too much in preparation for playing humans (which are still my main opponents).”
— Magnus Carlsen, in an interview with Peter Thiel at the Churchill Club: https://www.youtube.com/watch?v=ZBnSU-LX1ss&t=23m51s