It's definitely a useful trick for only showing certain content to certain users. For example, I wrote a script so you only show Digg buttons to visitors that have been to digg.com: http://int2e.com/blog/improved-digg-integration-script/

You don't even need Javascript.

    <a class="digg" href="http://digg.com/submit/">...digg button code...</a>

Now you can use CSS to hide visited links.

    a.digg:visited { display: none; }