I went from an XT to a 40D and would do it again in a heartbeat. High ISO shots look much better, you add spot metering, and I believe the processing time is much improved.
Those things are all great when it comes to affecting the final product, but what really does it for me are the things that help usability: The viewfinder is much larger. The ergonomics of the camera (wheel/jog dial) are better. And Live View is surprisingly handy.
There is a tradeoff of increased weight, but it's well worth it in my opinion.