As you all seem to have realised justified text is often more aesthetically pleasing. As for usability it can both help and hinder.
Justified text (i.e. justified both left and right) enforces the straight lines of a grid-structured layout, visually reinforcing the grid, and so supporting both the usability and aesthetics of the page.
The thing is this is all about the page seen as a whole like when scanning or browsing, but not that useful if you actually have to read the text, in which case ragged right hand side can actually help you keep your place in the text. (you automatically recognise the shape made by the different line lengths, to some degree, as you read down the page).
Not only can justification remove the benefits of ragged text, it creates more problems, for example the speed your eyes need to move across the page varies as the word or letter spacing grows and shrinks; or extra uneven lines of white space run though your block of text like rivers.
The issue on the web is compounded (as usual) because web browsers don't have the best justification algorithms (at least not yet). It can certainly make a mess, I think one of the biggest mistakes is the lack of auto-hyphenation when justification goes over-the-top with the squashing or stretching.
Well justified text blocks can look good and emphasise the important lines and structure of the page but badly justified ones undermine it and are distracting or create blotchy effects (where the text creates light and dark areas (blobs of white space and blobs of dense lettering) where a reasonably uniform weight (shade or darkness) is desired).
Obviously the algorithms will vary from browser to browser too, so you should check out how it looks on the browsers that most of your users use.
Remember the fact that reading on computer screens is already harder than reading on paper and you should see it's worth having second-thoughts about using justification. Also some people who already have trouble reading will often find justified text even harder. (standard reference web site: RNIB, page:
http://www.rnib.org.uk/xpedio/groups...xtjustify.hcsp)
My advice as you'll hear often, is to look at each case individually, with an informed mind and an appropriately balanced and objective viewpoint.
One way to guard against the problems of badly justified text is to avoid narrow columns which greatly reduce the chances of any given line of text having a good fit.
Another solution is to not use justification, replacing the straight vertical line made by the last letters of each line by an actual straight line, or the right hand edge of a block of colour. Sometimes this can be the only option if the medium of the web is encumbering your visual design and you can't change it (for whatever reason) to better suit the medium.