In December 2005, the science magazine Nature conducted a study to determine whether Wikipedia was as accurate as traditional encyclopedias, namely the Encyclopedia Britannica.
In Study: Wikipedia as accurate as Britannica, Daniel Terdiman summarizes the study results:
Wikipedia is about as good a source of accurate information as Britannica, the venerable standard-bearer of facts about the world around us, according to a study published this week in the journal Nature. ...
For its study, Nature chose articles from both sites in a wide range of topics and sent them to what it called "relevant" field experts for peer review. The experts then compared the competing articles--one from each site on a given topic--side by side, but were not told which article came from which site. Nature got back 42 usable reviews from its field of experts.
In the end, the journal found just eight serious errors, such as general misunderstandings of vital concepts, in the articles. Of those, four came from each site. They did, however, discover a series of factual errors, omissions or misleading statements. All told, Wikipedia had 162 such problems, while Britannica had 123. That averages out to 2.92 mistakes per article for Britannica and 3.86 for Wikipedia.
Of course, what makes Wikipedia different from a standard encyclopedia is that it can be updated by anyone and those updates appear immediately. When there's a particular hot topic item in the news, it's not uncommon for its Wikipedia entry to be updated rapidly, both by level-headed and factual participants and by more extreme, agenda-driven actors on both sides of the issue at hand. Consequently, when you read an entry at Wikipedia it's important to bear in mind that what you are reading now might include some dogmatic if not outright incorrect statements.
As Bibhas noted, it's important to check the sources and references. I also would encourage you to note the history of the piece you are reading. If there were many edits in a short window of time that may be a sign that there is some back and forth going between two "sides," which could imply that the information presented might not be as objective as it was prior to the volley of edits.