A few months ago I switched to Yahoo! Search as an experiment. I'm quite confident that, if you'd replaced the Yahoo! branding with Google's, I would never have known the difference.

But yet I switched back. Why? I was hosting the Silicon Valley Rails meetup, and our speaker Emmett Shear (of Justin.tv) had forgotten to bring his laptop, and had to borrow mine. I was so uncomfortable at the idea of my home page---Yahoo!, of all things, and not Google!---being broadcast to a room of Rails hackers that I changed it back to the Canonical Choice. And now inertia has returned, and I'm back in the warm bosom of mother Google.

I'm pretty sure that Yahoo! Search and Live Search are basically as good as Google nowadays. When it debuted, Google was qualitatively better than any other search engine, but today Google's lead is based mainly on branding.