Very fast in different aspects, fast AF, fast shooting. Sometimes I don't know if he AF can keep up with the machine gun alike shutter speed.
The IQ can be outstanding. My old 5D doesn't seen the light anymore since I've the lucky owner of this 1D. (The IQ of the 5D is on a high level too I know, but...)
However, an important disadvantage is the lcd screen, you can't rely on it for judging the sharpness of a shot. At home I'm often surprised by the quality of a particular image.