I think the Overture tool is useless. I’m pretty sure the search counts include people using the keyword search tool, that is: Webmasters. The counts are likely useless.
For example a couple years back, i entered ads for about 20 vioxx terms that supposedly had about 50-60,000 searches a month. After a month on the site, the impression (not clicks) stats for my ads, all on the first page of results was less than 1000 impressions for all the terms. So in a couple months they went from 60,000 searches to 1000? I don’t buy it. Is it possible that webmasters selling prescriptions might have searched 59,000 times for those terms? I think so.