In any market where the leader has a monopolistic marketshare it is a great idea to encourage innovation elsewhere and promote further competition. In the past Blekko was a great SEO data source but I couldn't use it as a default search service because the auto-firing of their slashtags were in many cases too restrictive. They did a recent update which still fires slashtag results, but now rather than requiring results to be part of that slashtag they allow the slashtags to compliment their search results.
That change has put their relevancy on par with Google & Bing for many head search queries, though a larger index size would likely help them score better on tail search queries. To help end users compare the results from the 3 leading search engines Blekko launched a meta-search service named 3 engine monte, where you can do a blind taste test of the search results from all 3 engines side-by-side.
The 3 engine monte tool is a great way to troubleshoot SEO issues, allowing you to quickly see if you are having issues with a particular search engine, or if the problem is something happening across the board. It is also a useful tool for checking out some of the algorithmic differences between search engines to understand how things like the Panda/brand layer impacts Google.