I absolutely HATE supporting IE < 9 designs. It's tedious, frustrating and no fun at all.
I'm designing for HTML5 and CSS3 (the "new" web is here now) and nearly always use the new features both language upgrades bring to modern, sensible browsers.
Modernizer To the Resque!
At first, I took it's promise for automagical compatibility improvements with ehem… older browsers… at trust… but then I fired up my virtual machine and saw the real difference.
Below are 3 pictures of a design I am creating in HTML based on specs from the designer. You decide: is Modernizer 2.0 worth the 10 kilobytes of JS?
(Above) Firefox 6 - How It Should Look
(Above) With Modernizer
Include CSS reset and HTML5 Boilerplate (or similar) in your design. Really, it helps. A lot.