To the modern web user, passwords are usually a nightmare, especially with the modern trend of “your password must contain every possible category of keys”. Well, how effective is that sort of thing?
(By the way, a measure of a great publication of any sort is when you end up “liking” and “sharing” most of its content. XKCD is one such.)
Of course, the flip side of the argument in the comic is: most users don’t think of random sequences of words, but most often come up with the easiest-to-guess phrases. In this case, some security is better than no security.
(Oh, and don’t forget to mouseover over the comic itself. And do that over every xkcd comic. You’re welcome.)