I prefer Google but Yahoo sometimes help in providing good keywords attached with your query. Also on Seo point of view Yahoo has a slight advantage as it also considers meta descriptions.
I use yahoo for an email account, YM and Yahoo news only, Google is still the best in term of search engine.
It seemed that almost everyone here are the same with what i do: Yahoo! Messenger and Daily Search Ranking. Well, google should push through Google Talk.
Don't use Yahoo anymore. Whenever i need to go for legal site reference for online shopping purpose, i either use Google or Bing
Being an SEO I use it to see how my site is doing in yahoo serp. I am getting good traffic from yahoo and that is one of the reason i sticked to see my rankings on yahoo. Mainly i use siteeplorer to see back links of my competitors.
I hit Yahoo's main page everyday several times to read news articles. They usually have something worth reading. I don't use the search engine though.