I don't think Yahoo is underrated as a search engine. Google is really the top one and deserves majority of the focus, in my opinion. If ever it were underrated, I'd say it would be just in very specific countries or specific industries depending on your target. But as for a general sense, I think Google is really where the focus should be, no question. I do like Yahoo as a company and like the services they provide however, like Yahoo! News and Yahoo! Answers, but as a search engine, I have a hard time imagining why anyone would choose to use it over Google.