As a secondary source? Damned appalling. However Wikipedia is not a secondary source, so that's not fair to judge it as one. As its actual classification as tertiary source, it's often pretty good, depending on the subject. Wikipedia on the back end is organized into little fiefdoms of users who make it, and if you hit on a good active fiefdom, it can be pretty good. Please check this comment out for a longer intro.

In some very rare cases involving contemporary sources, it can be acceptable - for example, back in the day H. P. Lovecraft mined the article on the Occult from the Encyclopedia Britannica (9th edition) when writing his story "The Horror from Red Hook," so if you were writing about that story, you would cite the encyclopedia - in much the same way, if somebody depended on Wikipedia as a source (or, more commonly these days, plagiarized parts of it wholesale and passed it off as their own writing) - you'd cite the relevant Wikipedia article (and probably a specific revision, given how often Wikipedia is freely edited).