This is Canon's highest performance professional midrange zoom for film, full-frame and 1.3x digital cameras. The Canon 24-105mm f/4L has great optics and works extremely well. My biggest whine is that the zoom ring takes a lot of effort and that the wide end of the zoom range is cramped, making it difficult to set wide focal lengths precisely.
It's silly to use this on a 1.6x sensor camera like the Rebels and 30D because 1.) you're paying a premium for large-sensor performance but using only the center of the image, and 2.) 24mm isn't wide on a 1.6x camera: 24mm on a 1.6x camera gives the same angle of view as a 39mm lens does on a 35mm camera. For 1.6x cameras, the excellent 17-55mm f/2.8 IS makes more sense because it weighs less, is one stop faster, has much less distortion and covers what I find to be a more useful range of focal lengths. Read the very nice article here