Our librarian goes on and on and on about how "Wikipedia is NOT an online encyclopedia."
And yeah, most of my teachers think this site's great.
I actually first came here after a teacher recommended it.
Metaphorically, I view Wikipedia as a city. You can either visit the city on a short vacation to find out information (aka Wikipedia), or you can ask a few intelligent residents who have lived there and what their thoughts of that city were. Mostly those residents will give an accurate impression of the city. These intelligent residents represent the contributers to book encyclopedias, like Britannica.
However, Wikipedia is a bit different. Essentially it allows all members of the community to communicate ideas, as long as they are bothered enough to do so. A lot of cases where people write Wikipedia pages is because they have a passion. The majority of articles written on subjects like mollusks, or tulips, or Broadway musicals are written by people with an interest in these specific areas - yet this also risks that such information may not be valid, or factually supported.
Sometimes Wikipedia experiences vandalism (akin to graffiti and crime of the city) and attempts are made to clean up errors, omissions and maintain validity of the articles through moderators (akin to local governments and government officials).
Personally, I'd rather read an article for a generalised idea on a specific issue or topic (such as Les Miserables, a novel written by Victor Hugo) through Wikipedia, than read a half-hearted article about the novel from by an employee hired for an encyclopedia company who had to write all the articles on literature of the 19th century and who might not even have a specific interest in the novel itself.
Though, it should still be noted that actually researching your own information first-hand (akin to living in the city yourself) is a better alternative than either Wikipedia or a static encyclopedia.