I always comment at least ONCE when I finish a good fanfic. If I didn't like it I usually tend to quietly 'leave' within one or two days. If I suddenly don't like it anymore halfway I still tend to comment on what I liked and why I don't like it anymore. But... in a respectful manner. Readers sometimes have no idea how much writers love it when people leave a comment on their story. It motivates them to update and keep going. Even if its a small comment or a slight criticism... its okay. At least people cared enough to comment.