Toby's First Law of Wikipedia

The utility of a page on Wikipedia is inversely proportional to the number of links on that page that you must click on in order to be able to understand the original page.

In practice, this means that it will take a very long time to learn a new (to you) field from the pages on Wikipedia (especialy math) but that the utility of Wikipedia shoots way up when you are researching in your area of expertise or closely related fields.