To me, a well-written movie review does three things: 1) Gives a good idea of what the movie will be like, 2) Gives just the right amount of plot detail
without giving away too much, 3) Gives new insight into the film when read after viewing.
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Ebert does pretty well at #1 and #3, but he is terrible at #2. He gives far more plot detail than anyone should care about
before seeing the movie, and he sometimes gives away surprises.
Imho, the
AV Club reviewers get all three exactly right, and are often quite funny as well.