Issues with Wikipedia
Because all Wikipedia content is user-generated and there is no central authority vetting or managing this content, some experts dismiss the usefulness of the site. Issues with Wikipedia fall into two general camps: accuracy and depth of coverage.
If anyone can write or edit an article, how are you to know if the submitted information is accurate? Although the Wikipedia community is self-policing (and the information generally accurate), misleading or just plain wrong information can seep into the site. It is possible for mistakes to creep into Wikipedia's content and not be discovered by the base of contributing users—and for those mistakes to be reflected in papers and reports written with Wikipedia as the sole source.
Depth of Coverage
Because Wikipedia users suggest the content, it's likely that some popular culture topics are more covered in more depth than topics of a more intellectual bent. This is solely a function of which and how many contributors are interested and expert in a given topic. As such, you can't depend on the Wikipedia to always provide adequate content.
Beyond the Issues
With these issues in mind, it's best to view Wikipedia content as a start, rather than the final word when researching a topic. When you're writing a scholarly or professional paper, or just digging up information on a given topic, you should not use Wikipedia as your sole source, but rather as a guide to additional sources. In addition, it's always a good idea to check the footnotes and other references in a Wikipedia article to confirm the source of information presented; the most accurate articles are well sourced.
Despite the potential issues, Wikipedia remains the information source of first choice for millions of users. It's a great place to find information on just about any topic, from the highly academic to the casually popular. Whatever you're looking for, chances are someone else has written about it on Wikipedia.