A WebmasterWorld thread has webmasters and SEOs discussing the SEO research benefit by using Google's similar or related search functions.
Most SEOs and Webmasters feel the similar results are mostly random but they can help SEOs and webmasters by maybe understanding how Google categorizes your site, if Google puts you in a "bad neighborhood" and see which sites have a similar link structure to you.
But more SEOs and webmasters play more weight on the related command.
Robert Charlton said the "related:example.com/pagefile search used to return domains/pages that shared an inbound link source with the url being checked." Some say it is a good way to see the link sources you should be targeting.
How do you use these search features?
Forum discussion at WebmasterWorld.