Wikis: Friend or Foe?

Wikis are a bit of a strange subject for me. When i was doing the Library Tech's Course, they told us not to use wikis as a source of information as anyone can go on and add anything they like about any subject. For example, i could go on and say that i am a scientist and i've just discovered that there are 20 planets in the solar system and make up some names for them. What if a student copies that into an assignment? On the other hand you can access up to date information without having to wait for the next edition of Britannica to be published. Hmm, very difficult to decide. Friend or Foe?