Shally and I have long advocated that you want to check multiple search engines for any given query, because the results can vary dramatically, even for the exact same search string. Therefore, you'd be missing out on great results if you only depended on one. In 2005, I attended a web conference where I discovered Thumbshots Rankings, which allow you to graphically see the low overlap between search engines which proves this -- and we've been using it in webinars ever since!