The site is most useful when you know either absolutely nothing about something, or a great deal about it. It permits those utterly unaware of a topic to get some essential facts – probably true – very quickly and easily. It also allows real experts to track down something they once knew, can remember, but had forgotten very quickly.
Wikipedia is least useful for those in the middle zone. These are people who know more than the minimum, but not enough to really judge the credibility of complex arguments in the subject area.
Nonetheless, it is a wonderful resource. I use it at least twenty times a day.