We recently created an online survey on the accuracy and completeness of Wikipedia articles.
So far, 50 respondents have evaluated a Wikipedia article in an area of their expertise. Of those, 76% agreed or strongly agreed that the article was accurate and 46% agreed or strongly agreed that it was complete.
Of the 50, 18 compared the article they reviewed to the article on the same topic in the Encyclopedia Britannica. Thirty four percent of those people found the Britannica more or substantially more accurate and 39% found the Britannica article to be more or substantially more complete.
Please complete the survey yourself (for as many articles as you wish) and forward the link to others.