I've always tried to make my web site flexible so I could change the look on a whim. But, I've always used tables to do layout. Not any more! This site is now running entirely on CSS. I only use tables where tables should be used: for, well, tabular data. Also, all menus actually make sense if you don't have the style sheet. For example, the navigation menu to the left looks like a bulleted tree if you don't have the style sheet.
Why? Why not? Actually, Randy put the bug in my ear with his post on clever style sheets. The flexibility that sites like http://www.csszengarden.com show is really amzaing. Of course, that site has a single page with no dynamic content, so it's pretty hard to make everything that flexible. We'll find out, at least, as I modify this site's look and feel over time.