Thumbshots has a useful tool that lets you easily and graphically compare the overlap between top results for searches on major search engines such as Google and Yahoo. SearchEngineWatch has a good article with details.
Your conclusion might be that this proves the value of metasearch engines, but that's not obvious. The data only shows little overlap in the top search results; that is, search engines differ in how they prioritize search results. When combining data from multiple search engines, the trick is to find a combination that results in a better prioritization than any of the individual search engines. That's very hard to do because the metasearch engine is working with less data (often just the page summary) and usually has to do the prioritization in real-time.