With all the excitement about Microsoft finally developing a competitive search engine, test and vote for which results you prefer.
For mine, I searched for "Tim Ferriss," probably not the best query for a test like this, but whatever. Yahoo wasn't even close (results included the official site of INXS and Youtube channel of user "masterlock77"), but the other two were tough. I ended up choosing Google's results because Bing seemed a little redundant (three or four of Tim Ferriss's blogs, maybe not necessarily Bing's fault), and Google's had a link to an interesting post on his blog, as well as another interesting blog post comparing Tim Ferriss and Gary Vaynerchuk.
All in all, I'd say Microsoft has done well with Bing. Google has always done a good job with its search, but it's a good (for us, at least) that they now have a close competitor.
For these keywords I liked Google's results best, then Yahoo's, then Bing's. I thought the quality of Google's and Yahoo's results for these keywords were about the same.