I love this camera! I had an original 1D, sold it when I got the 1DM2. Had a 10D, then a 20D as smaller, lighter backup. When shooting both the 1DM2 and the 20D, I got better color with the 20, and had to work hard fussing with white balance of the 1DM2 raws to get the images to look like they came from the same shoot. Also, the 1DM2 gave me a fair number of OOF images even in 1-shot.
I've had no AF problems with the 1DM3. AIServo is fast and accurate. ISO 6400 is amazingly good. I am considering buying another 1DM3 and selling the 20D.