Copyscape: effective plagiarism-preventing service or not?

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I came across a "Protected by Copyscape" banner in a writer's blog and was curious enough to check out the service. From their About page:

Copyscape is dedicated to protecting your valuable content online. We provide the world's most powerful and most popular online plagiarism detection solutions, ranked #1 by independent tests. Copyscape's products are trusted by millions of website owners worldwide to check the originality of their new content, prevent duplicate content, and search for...