While conditional comments can be handy when you're really really stuck, I don't like to rely on them - mostly because CSS best feature is seperating style from content, and conditional comments must be in the HTML. You can change the entire appearance of a site by editting one CSS file - but if you're using conditional CSS you may have to edit many HTML files. There are other methods of limiting CSS to IE.
/* display for IE only */
* html .whatever {
font-weight: 300;
/* hide from IE mac \*/
color: #ff0000;
/* */
}
Most CSS problems between IE 5, 5.5 and 6 on windows are easily resolved. If you have a particular problem that needs solving, let me know
-- update: to answer demerph --
See 398203