This is an old 2004 list of content reviews of Wikipedia. The current two entries are both third-party reviews which also compare wikipedia's content to that of other encyclopedias. This page need not limit itself so narrowly; if you know of other content reviews of any kind, please list them here.
CD release review, October 2x 
Die Zeit, October 14? 
In mid-October, the German Wikipedia underwent its second professional content test, this time overseen by Die Zeit, a leading weekly newspaper in Germany's. Result: de:Wikipedia, the "popular dispenser of knowledge on the Internet", although filled with "deserts of text", is HOT. While it is "completely without multimedia" and "does not support complex search queries", it "stands in first place when it comes to text content," receiving top aggregate marks in three broad content categories -- Natural Science and Technology, Humanities and Social Science, and Culture. Moreover, Wikipedia's "lead in current events is a mile wide".
This review was longer than the c't review -- seven terms in each of 21 fields -- but the article was shorter and less detailed. Die ZEIT was kind enough to extend the comparison to include both smaller German encyclopedias (Data Becker, aimed at a younger audience, and Universallexikon) and the English-language Britannica 2005 DVD. Britannica, she of the "legendary 32-volume set," took top marks in the sciences, but fell down when it came to sports and, most notably, current events. All the same, "the 165,000 well-sorted Web links alone are worth the price".
As to that perennial bugbear, editorial responsibility, the reviewers addressed it directly, if vaguely:
"Those concerned with the quality of [Wikipedia] articles, because no established editorship takes responsibility for them, can rest at ease: we had most of the Wikipedia articles we examined judged separately by specialists in their respective fields, and they were thoroughly done. The texts still have gaps here and there, but they make up for it elsewhere with precise and detailed descriptions. And everyone can engage himself as a gap-filler: clicking on the link "edit this article" makes the reader an author."
Original article : http://www.zeit.de/2004/43/C-Enzyklop_8adien-Test
c't review, Oct 2 
In their early-October edition, c't, the popular german magazine for computer engineering, released a study they conducted of the three major digital encyclopedias in germany -- Brockhaus, Encarta, and (most recently) Wikipedia. The article was titled Encyclopedias: Wikipedia vs. Brockhaus and Encarta, and found on pg. 132 of the print magazine.
c't tested the encyclopedias on breadth, depth, and comprehensibility of content, ease of searching, and quality of multimedia content.
The content test was the most elaborate : first they divided content in three broad fields, Science, Society, and Culture. They further subdivided these into 22 total subject areas, and within each subject selected an easy, a moderate, and a difficult topic. They then searched for the best matching article (and supplementary content) in the encyclopedia.
Finally, they brought in experts in each broad field who rated the articles from 1 to 5, based on technical correctness and completeness of the texts, and on their comprehensibility. Once this was finished, the results were totalled at each level of conceptual difficulty, within each broad field, and across all 66 topics.
The net result: Wikipedia took the top prize, a comfortable distance ahead of the other two. "Brockhaus Premium surpassed the competition from Redmond," the review reported, "but must however concede defeat to Wikipedia".
Happily, the full breakdown of the experts' ratings were published along with the article, so that each encyclopedia may benefit from the spot check.
You can find the website for the magazine online at http://www.heise.de/ct/ .