Amazing how many people just slap a keyword search interface on their site and call it a day... They'd be better off pointing at Google rather than rolling a crappy home grown solution.
Good post explaining table stakes for reasoning about whether your site search works (and measuring whether changes make it work better).<p>If anyone is interested in having this but not rolling their own, www.switfype.com (ycombinator 2012) provide a pretty powerful site (and mobile app, etc) search solution, which includes these kinds of analytics and more (disclaimer: the founders were my roommates :-)).