New study in Nature magazine evaluates 42 science entries in both sources, checks for errors.
“Only eight serious errors, such as misinterpretations of important concepts, were detected in the pairs of articles reviewed, four from each encyclopedia … but reviewers also found many factual errors, omissions or misleading statements: 162 and 123 in Wikipedia and Britannica respectively.”
Best comment from Digg coverage of this article: “What I want to know is after they found the errors in wikipedia, did they bother to fix them?”
Related: Best way to search Wikipedia – Wikiwax.com.