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WebGuerrilla - 1:51 am on Feb 20, 2001 (gmt 0)
Other than giving you a good snap shot of overall search trends, they aren't much help for specific SEO projects. They only contain top level, usually single words that are far to popular and broad to be the basis of your SEO efforts.
Both WordSpot and WordTracker (subscription based services) provide top 1000 and top 500 search reports. I'm not sure if they still offer it, but WordSpot used to have a free email service that would send you the top searches each week.