A couple studies have shown that Wikipedia is, on balance, about as accurate as "real" encyclopedias. The fact that anyone can edit anything is a sticking point, but it generally takes only a matter of minutes for someone to notice the change and set it back. I've read thousands of Wiki pages, and I've never actually noticed any misinformation.
Its biggest strength is in technology matters, because the internet is the only form of media that's really up to the task of keeping up with the fast pace of advances. It's also good for uncontroversial matters of science, because it tends to be far more exhaustive on a subject than a traditional encyclopedia.
On contraversial matters, it tends to fall prey to the idea that both sides should be presented with equal weight, even if one side is disputed by 99% of people. It tries a little too hard to be fair and balanced on these issues.
It's still considered poor form to use a Wiki article as a formal cite for a paper, but as a general resource for, say, people in an internet debate, it's pretty solid. It's at least reliable enough that if someone posts something to demonstrate a point, it's pretty flimsy to simply say, "Oh, that's just Wikipedia, I'll ignore it" without at least posting a counter-cite.
