Full justification on the web is usually a bad idea.
Typographers use full justification to get an elegant-looking block of type. The straight right edge is a strong visual element on the page, and can add to the controlled overall look. But typographers care about more than just the outline of the rectangle. They care about the evenness of the type within the rectangle, something they call "color". The goal is to get an evenly filled area, with no large changes in density.
Because full justification involves stretching word spaces, if a line has to be stretched too much, the spaces become wide enough to be noticeable white blobs on the page. The line of text is then "too loose", and interrupts the flow of reading.
In traditional typography, hyphenation is used to reduce the need to make loose lines. By breaking words into smaller chunks, the lines can be filled more naturally, and they don't have to be stretched too far.
But web browsers don't hyphenate. As a result, paragraphs often suffer. Here are some examples from the OpenID news for February 2008:
(I've blurred it a bit to emphasize the color.) This paragraph is OK, with just two problem lines: the fourth ("some of the top ...") and fourth from the bottom ("to support the community ..."). These lines are loose enough that I stumble when reading them, as if they were typed "some .. of .. the .. top .."
But then we come to the other problem with full justification on the web:
Occasionally URLs appear in paragraphs, and these are very large "words" that completely screw up the line before them. Technical writing is especially prone to this as other non-word content appears in running text, such as function names.
I think full justification is one of those technology hold-overs: the new technology trying to mimic the old. Books and newspapers use full justification, so we try to do it on the web also. But content on the web rarely appears in a constrained rectangle. Full justification in print is appealing partly because the justified right edge of the text is a good echo of the right edge of the paper, or of the left edge of the next column in a newspaper. In a single column of text in the middle of a browser window, full justification isn't gaining you much, and brings you pain in the form of loose lines.
Except in specialized cases, or where you know very clearly what type of content will appear, you shouldn't use full justification on the web. The lack of hyphenation is a killer.
As it happens, there are browser-side hyphenation solutions, but they also have their drawbacks: code size and execution time.
BTW: it isn't just the web that suffers from hyphen-less justification. Amazon's Kindle has the same problem, something I noticed right away when I first tried one out. I'm not sure why they wouldn't have built hyphenation into a reading device. And I'm reading a Salman Rushdie book published by Penguin which uses no hyphenation. Why would a traditionally-published book forgo the tried and true technology of good-looking pages?