Google has optimized AdSense for search. Now, it's powered by Custom Search which is much better (looking and functionality). I have replaced AdSense for search with Google Custom Search because I liked its look and how much you can customize Google Custom search but I had to manage two accounts one for search and one for AdSense now, I don't have to! Google updated its AdSense for search to Custom search. Great Improvement! Now you can also save your settings on your account and change them without having to re-enter the code in your site (same like AdSense for content) Look here for all the benefits: http://adsense.blogspot.com/2008/05/adsense-for-search-now-powered-by.html
IMHO the search box they give you for adsense leads to results that look like crap. I wish I could just have them sent directly to a page that looks exactly like a google search page... They make billions of dollars. I'm certain that the pages they use on their own site are designed to lead to the best outcome (user experience and the mighty dollar)..