Which search engines need tuning?
The number of interested visitors a search engine sends to your web site is more important than the sheer number of visitors a search engine sends to you -- especially if those users were delivered via a pay-per-click program (and not just a straight web search). Interested visitors -- visitors who spend a lot of time on your web site -- are more valuable than uninterested visitors. The most valuable search engine campaigns are the ones that send you large numbers of users who spend lots of time at your web site.
Used in conjunction with the Average Stay Length per Search Phrase report, you can see which keywords are the most effective in bringing useful visitors to your site. Keywords that bring visitors that stay the longest are likely worth more money than keywords that don't bring interested users.