A couple weeks back we mentioned that Google's Peter Norvig stated that Google does not use search usage data directly in their relevancy algorithms. Yesterday Matt Cutts made a post on the official Google blog stating that Google does look at search logs / usage data to determine how large spam attacks are and how well new anti-spam measures are doing

Data from search logs is one tool we use to fight webspam and return cleaner and more relevant results. Logs data such as IP address and cookie information make it possible to create and use metrics that measure the different aspects of our search quality (such as index size and coverage, results "freshness," and spam).

Whenever we create a new metric, it's essential to be able to go over our logs data and compute new spam metrics using previous queries or results. We use our search logs to go "back in time" and see how well Google did on queries from months before. When we create a metric that measures a new type of spam more accurately, we not only start tracking our spam success going forward, but we also use logs data to see how we were doing on that type of spam in previous months and years.