Wikipedia is a great source of information. It contains over 3 million articles about various subjects covering almost everything you could be interested in. The content is written by people like you and me, by professionals and amateurs, experts and novices. In addition to creating content, the information is watched, edited and revised to improve continually the quality of the articles. Various studies confirmed Wikipedia's accuracy to be as good as the leading paper encyclopedia Encyclopedia Britannica.
Yet, there are people claiming that Wikipedia shouldn't be used a valid source of information while Britannica is perfectly valid. Both encyclopedias contain errors in their articles, although they are usually minor mistakes. The most common fear about Wikipedia is that anyone can edit an article which means vandals could write false information (and it happens quite often). Such acts of vandalism are usually reverted quite rapidly, but what if you got your information while the wrong information was there?
The truth is that you can't trust anyone! No single source of information is perfectly reliable, scientists publish fake data, vandals like to prank people and people's opinions are biased. When looking for information, validate that information with multiple references and most of all with your common sense. One last thing, when looking for references to confirm your information, careful about circular references which is a common mistake for users of Wikipedia.
This is how it usually works (from Slashdot):
Wikipedia states a false fact, a reputable media outlet copies the false fact, and this outlet is then used as the source to prove the false fact to Wikipedia.
Wikipedia is not the most reliable source, but any other source is just as likely to be wrong.