I have two new 1D Mark 4 bodies which arrived last week. I am upgrading from the 1D Mark II; my current cameras are five years old.
Not having had the new cameras very long, I've noticed a few things:
Focusing with my fast primes is significantly improved over the 1D2 bodies. I shoot a lot wide open with the 24/35/50, and this camera nails the focus in a much higher percentage of frames. (Note that I generally use the center point and recompose. That's a habit forged over twenty five years of shooting, and one that I am not likely to change.) I'm not a sports specialist, but in general photo-J use I am getting more in-focus than I did with the 1D2 bodies.
High ISO is good, maybe a 2 stop improvement. With the 1D2, ISO 800 looked good, and ISO 1600 was just usable. With the 1D4, ISO 3200 looks good, and ISO 6400 is usable. (Yes, I've seen samples from the D3/D700, and the Canon files aren't as noise free, but the level of detail is very good.)
The files can take a LOT more punishment in the raw converter and still look great. Which is good, because I still haven't figured out the histogram and the LCD screen -- photos that are perfectly exposed when I check the camera are a stop or more underexposed when I open them in the raw converter. (With the 1D2 there was a perfect correlation between the histogram, LCD, and raw files.) Gotta work on that one...
Overall I feel pretty good about these cameras.