News article just the other day that I read said that when compared to encyclopedias like Britannica, which had something like a 95%+ accuracy (I don't remember if they discussed methodology, but it might have just been comparison to other encyclopedias. I think they mentioned two or three others), Wikipedia had about 85% accuracy.
Personally, I use it as a starting point whenever I need to do a research paper or the like- read the Wikipedia entry to get an idea of what I need to be looking for (e.g., major moments in, say, William Faulkner's life, or important factors in the outbreak of (war of your choosing here)), and usually the sources at the bottom are fairly reliable. But I never quote/cite Wikipedia because A) my teachers would not count that as a source and probably make everything I cited from it count as uncited evidence, and B) whatever I quote might not be there a week later or whenever the teacher is checking quotes/sources, and quoting something that's not there or if the quote isn't the wording used because someone changed it then that looks bad.
By and large though I believe it's fairly reliable, but not a good place to do serious research.
EDIT: Here's a few links on the subject.
http://news.cnet.com/Study-Wikipedia-as-accurate-as-Britannica/2100-1038_3-5997332.html
http://news.bbc.co.uk/2/hi/technology/4530930.stm (basically same story as above)
http://news.bbc.co.uk/2/hi/technology/4840340.stm (criticism of the study)
http://en.wikipedia.org/wiki/Reliability_of_Wikipedia (dohohohoho see what I did there?)
Can't find the article I read that mentioned 85% though.