I see a lot of sites using Google’s AdSense search over WordPress’ built in search or some other cms, forum or blog app’s built in search. I personally think this is bad for the user. Google is great for searching on the web, but it simply doesn’t find everything on a website. Google’s Site Search should only be used when you do not have a search feature at all. Even on big sites like Digg or Smashing Magazine I can’t get good results from Google using the “site:url query” method. I know its missing a lot of pages in it’s results. I understand people use it to help monetize their site, but I think it comes at the cost of hurting a user’s ability to find what they are looking for on your site. The fact is Google doesn’t have all the pages of a web site indexed, so a web app’s built in search will always be better than an external search.