If you liked the story, say so. It's not required,though. It may be you liked some element of the story but not all of it. Perhaps the last time you read this author you noted his/er characters were not well developed, but were on this particular story, say so. I am always delighted when someone mentions what they particularly liked. I don't mind if another reader disagrees. They are opinions, not truths.
canadianspankee:
Should one always say whether they liked or did not like the story, or is a comment on the story enough?