Can't remember where I read it, but it said something like most Wikipedia "vandalism" was corrected within four minutes.
Remember, Wikipedia has checks and balances for writing to it. I even corrected something once.
I was reading the entry on the Toronto Maple Leafs and some clown changed the names of The Kid Line to Chen Lee, Kwang Ho and Kam Lo (making names up, can't remember what they were exactly) I figured out how to make a correction and went and fixed it.
It's been so long, but IIRC it logs people's IP addresses so they have an idea of who is vandalizing and who is correcting, so vandalism is kept to a minimum. In fact, I don't recall seeing anything glaringly wrong since (not to say that doesn't happen).
To me the strength of Wikipedia are the links at the bottom of the entry that lead to a more first hand source of info.