Not that I'm a designer or know what I'm talking about, but a few years ago I switched from doing all of my graphics work in The Gimp to all scalable vector graphics in Inkscape, and couldn't be happier.

It's a different method altogether, and skills from one don't pass to the other, but once you get the SVG tricks down you can do a lot of tremendously complicated designs with little effort, and the master copy is a breeze to work with. Modifications and post-processing manipulations become cheap operations.

It's a little unstable outside of Linux, but does run on OS X and Windows. On OS X, it crashes pretty often but recovers cleanly.