Measuring the success of your search solution

During the event on June 7th, 2011, a good discussion came up about how one might measure the success of a search solution. I would like to explore this further with the community. Some specific questions that might be used: * What makes a successful search solution? * How can you separate success of search from overall success of your site (really, how to separate success in search from success in navigation)? * What kinds of metrics might you use to measure success? Are there any? * What are some subjective measures one can use to measure your search solution? * Is it possible to define a universal framework for measuring success that supports comparison of different solutions?