Useful, expansive information, though at times not always correct. Any factual errors or bias is usually resolved quickly, and little "warning" tags are given to let you know if it is believed to not have a neutral stance, etc. Despite this, usually not suitable for citing school papers and the likes.
I think that many people see Wikipedia as this encyclopedia that anyone can edit, which would make it unreliable as a source of information. This is not the case. Most articles have a detailed section showing references and citations used in the creation of the article.