According to the American Customer Satisfaction Index Yahoo has edged out Google by 1% for customer satisfaction this year: Yahoo 79%, Google 78%.
The rating system the ACSI uses is complex and listed here and their source for data is noted here. Essentially their scoring focuses on: customer expectations, perceived quality, perceived value, customer complaints and customer loyalty.
See the chart below for more detail on the other search engines reviewed.
Thanks to Loren Baker at Search Engine Journal for the heads up on this interesting data.